DSSSB PGT Computer Science PYQ (Technical), 11 Jul 2021

Explanation

The program counter always holds the address of the next instruction to be fetched; it is incremented (or loaded with a branch target) after each fetch.

Explanation

An interface unit sits between the CPU and a peripheral and resolves differences in speed, data format, signal values and timing between them.

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On a data input command the interface takes the data received from the peripheral into its buffer register and places it from that register onto the data bus, so the CPU can read it.

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When BG = 1 the CPU has given up the buses and the DMA controller is the bus master, so its RD and WR lines become outputs that control memory reads and writes.

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In powers of 1024: yottabyte (2⁸⁰) > zettabyte (2⁷⁰) > exabyte (2⁶⁰) > petabyte (2⁵⁰) > gigabyte (2³⁰) > megabyte (2²⁰).

Explanation

Required capacity is 128K × 8 bits. Each chip is 16K × 1 bit, so (128K/16K) × (8/1) = 8 × 8 = 64 chips.

Explanation

The minuend is the number from which another is subtracted. Here (011100)₂ is subtracted from (1011100)₂, so (1011100)₂ is the minuend and (011100)₂ is the subtrahend.

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The control unit directs and coordinates every other part of the computer by sending control signals, just as the nervous system coordinates the body.

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An assembler translates assembly-language mnemonics into machine code. Compilers and interpreters translate high-level languages, and a linker joins object files.

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n bits can form 2ⁿ distinct bit patterns, so an n-bit opcode can encode 2ⁿ different operations.

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(10110)₂ = 22 and (10011)₂ = 19, so the product is 22 × 19 = 418. 418 = 256 + 128 + 32 + 2 = (110100010)₂.

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(3564)₈ = 1908 and (323232)₄ = 3822. The sum is 5730, and 5730 = 1×4096 + 6×256 + 6×16 + 2, which is (1662)₁₆.

Explanation

+28 in 16 bits is 0000000000011100. Inverting the bits gives 1111111111100011, and adding 1 gives 1111111111100100.

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For base 7, the 6's complement (diminished radix complement) is found by subtracting each digit from 6: 6666 − 1543 = (5123)₇.

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The OS keeps each process's state, program counter, registers, scheduling and memory information in its Process Control Block (PCB).

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Throughput is the number of processes completed per unit time. Turnaround time is the time to complete one process, and response time is the time to first response.

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Memory allocation and managing resources for efficient utilization are both core functions of an operating system.

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The four Coffman conditions are mutual exclusion (non-sharable resources), hold and wait, NO preemption and circular wait. Preemption itself prevents deadlock, so it is not a condition.

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Shortest Seek Time First is disk scheduling, Shortest Job First is CPU scheduling, Least Recently Used is page replacement, and the Banker's algorithm is deadlock avoidance.

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Semaphores can be implemented in software (e.g., with OS/kernel code or algorithms like Peterson's) and with hardware support such as test-and-set or atomic instructions.

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Best fit: 10 KB→12 KB (2 KB left), 2 KB→the 2 KB hole, 14 KB→15 KB, so all three fit. First, next and worst fit put 10 KB into the 15 KB block, leaving no block for 14 KB.

Explanation

FIFO with 3 frames: faults on 3, 4, 2; 3 hits; then faults on 9 (out 3), 1 (out 4), 6 (out 2), 3 (out 9), 4 (out 1), 9 (out 6). Total = 9 faults.

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A process goes from running to waiting only when it waits for I/O or for an event. A lower-priority arrival never affects the running process (a higher-priority arrival may preempt it to ready).

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SSTF from 50: 34 (16), 20 (14), 19 (1), 15 (4), 10 (5), 7 (3), 6 (1), 4 (2), 2 (2), 83 (81). The total movement is 129 cylinders, so it takes 129 seconds.

Explanation

By the consensus theorem, BC is redundant: BC = BC(A + A') = ABC + A'BC, which is absorbed by AB and A'C. So AB + A'C + BC = AB + A'C.

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NAND (and NOR) are universal gates because any Boolean function, including NOT, AND and OR, can be built using only NAND gates.

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Each of the n variables can be 0 or 1, giving 2ⁿ input combinations, so the truth table has 2ⁿ rows.

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In BCD each decimal digit is coded separately in 4 bits: 4 = 0100, 5 = 0101, 8 = 1000, 9 = 1001, giving 0100 0101 1000 1001.

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DCTL stands for Direct-Coupled Transistor Logic, an early logic family in which transistors are connected directly without coupling resistors.

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A POS expression is a product of maxterms (OR terms), and an SOP expression is a sum of minterms (AND terms).

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A decimal-to-BCD encoder has 10 inputs (one for each digit 0-9) and 4 outputs (the 4-bit BCD code).

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A demultiplexer takes one input and routes it to one of several outputs according to select lines. A multiplexer does the opposite.

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The 1-cells are A'B'C'D' and ABC'D' (CD=00), A'BC'D and AB'C'D (CD=01), and ABCD (CD=11). Adding these five minterms gives option (B).

Explanation

25 = (11001)₂. For 0.625: ×2 = 1.25 (1), 0.25×2 = 0.5 (0), 0.5×2 = 1.0 (1), giving .101. So 25.625 = (11001.101)₂.

Explanation

N select lines can address 2ᴺ inputs, so a multiplexer with M data inputs needs N select lines where 2ᴺ = M.

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An AND gate gives a high output only when all inputs are high, half a byte (4 bits) is a nibble, and a D flip-flop's output follows its D input on the clock edge.

Explanation

27 = (11011)₂. The Gray code keeps the MSB and XORs adjacent bits: 1, 1⊕1=0, 0⊕1=1, 1⊕0=1, 1⊕1=0, giving 10110.

Explanation

Y = (A + BC)(B + AC') = AB + AC' + BC. AB is the consensus term of BC and AC', so Y = BC + AC'. Also (A + C)(B + C') = AB + AC' + BC, the same function, so both are correct.

Explanation

Q0 toggles on every clock and Q1 toggles whenever Q0 goes 0→1 (it is clocked by Q0). From 00: Q0→1 so Q1 toggles (11); then Q0→0 (10); then Q0→1 so Q1 toggles (01). This is a down counter.

Explanation

Portability (running on different machines) is a desirable attribute of a good language, so non-portability is not one. Clarity, abstraction and ease of verification are all good attributes.

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In flowcharts a parallelogram shows input/output, a rectangle shows processing, an oval marks start/end and a diamond marks decisions.

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Machine language is hard to read and modify because it is pure binary and machine-dependent. So "easier to modify" is false.

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Breaking the grammar rules of a language (e.g., a missing semicolon or misspelt keyword) causes syntax errors, which the compiler or interpreter reports.

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Supercomputer speed is measured in FLOPS (floating-point operations per second), e.g., petaFLOPS.

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Form.StartPosition sets where the form first appears when it opens (e.g., CenterScreen or Manual), that is, its initial position.

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The Break All button pauses (breaks) execution of the running program so it can be debugged. It does not delete items.

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Control names must be one word made of letters, digits and underscores. "txtFirst+Last Name" contains a + sign and a space, so it is invalid.

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IDE means Integrated Development Environment: the editor, designer, compiler and debugger combined in one tool.

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In the Visual Studio/VB.NET debugger, F10 is Step Over (it runs a procedure call as one step), F11 is Step Into and F9 toggles a breakpoint.

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The Immediate window shows/evaluates current variable values, the Properties window shows the selected object's properties, Project Explorer lists forms and modules, and the Design window is where the form is designed.

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Object-oriented programming is organised around objects. Procedural programming is built on procedures, functional programming on functions, and logic programming on facts and rules.

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Modular programming breaks a problem into smaller, independent modules that are developed and tested separately.

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Breaking a program into modules makes it easier to use and maintain, and lets the modules be reused in other programs.

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Objects communicate by message passing, that is, one object calls another object's methods.

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A class is a blueprint, and an object is an instance of that class created in memory.

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A class created from an existing class by inheritance is called a derived class (subclass). The existing class is the base class.

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Inheritance, abstraction, polymorphism and encapsulation are OOP features. Abduction is a form of logical reasoning, not an OOP feature.

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Python is case sensitive (Name and name are different variables), so "case insensitive" is not one of its features. It is high-level, interpreted and portable.

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>>> is the prompt of Python's interactive interpreter (shell), meaning it is ready to accept a statement.

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Curly braces holding key : value pairs create a dictionary (dict) in Python.

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Statement II is true: assignment creates a variable and binds a value. Statement I is treated as not true because a variable can only be rebound; values of immutable objects such as strings and tuples cannot be changed in place.

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x //= y is floor division: 125 // 13 = 9 (13 × 9 = 117, remainder 8), so x becomes 9.

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The event queue holds incoming events and decides the order (sequence) in which they are dispatched and handled.

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'All the Best' has 12 characters, so index −3 is index 9. Indexes 8-11 are B, e, s, t, so str1[-3] is 'e'.

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In any binary tree, the number of leaf nodes is one more than the number of nodes with two children, so n leaves means n − 1 nodes of degree 2.

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Start→Next is the node holding 140, and its Next is the node holding 128, so the Info is 128.

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In a 1-based circular queue of size N, it is full (overflow) when Front = 1 and Rear = N, or when Front = Rear + 1 (rear has wrapped round to just behind front).

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In-order traversal of any binary search tree visits the keys in ascending sorted order: 10, 20, ..., 100.

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In an AVL tree, the heights of the left and right subtrees of every node differ by at most 1: |h(TL) − h(TR)| ≤ 1.

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If the item is equally likely to be at any position, the average number of comparisons is (1 + 2 + ... + n)/n = (n + 1)/2.

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For large n the growth order is log n < n < n log n < n² < 2ⁿ.

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Row-major with 1-based indexes, rows before row i hold 3(i−1) − 1 elements, and (i, j) is the (j − i + 2)th element of row i. Total = 3i − 4 + j − i + 2 = 2i + j − 2.

Explanation

The number of distinct BSTs on n keys is the Catalan number C(n) = (2n)!/((n+1)! n!). For n = 6 this is 132.

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Binary search needs at most ⌈log₂(n+1)⌉ comparisons. 2²⁰ = 1,048,576 ≥ 10,00,000, so the maximum is 20.

Explanation

177 mod 57 = 177 − 171 = 6 and 197 mod 57 = 197 − 171 = 26.

Explanation

The root is B (last in postorder). Left subtree {E,I,C,F} has root C, with E (right child I) and F. Right subtree {G,D,J,H,K} has root D, with left G and right H (J, K). Preorder: B, C, E, I, F, D, G, H, J, K.

Explanation

Pass 1 merges pairs: (200,470)(80,150)(40,90)(300,400)(70,120). Pass 2 merges runs of two into fours: 80,150,200,470 | 40,90,300,400 | 70,120.

Explanation

Selection sort always makes about n²/2 comparisons, scanning the unsorted part for the minimum on each of n passes, so it is O(n²) in every case.

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A namespace defines a named scope for identifiers, so names such as std::cout do not clash with the same names elsewhere.

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>> with cin is the extraction (get from) operator, and << with cout is the insertion (put to) operator.

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void is used as the return type of a function that returns nothing, and as in f(void) it shows an empty argument list.

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bool holds only two values: true and false.

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A function prototype states the return type, the function name, and the number and types of its arguments.

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The assignment operator is = and it associates right to left, so a = b = c assigns c to b first, then b to a.

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A static member function can access only static members of its class, and it can be called with the class name (Class::f()). So only I is correct.

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The ternary operator ?: is one of the C++ operators that cannot be overloaded, along with ::, . and .*. The other statements about it are true.

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In C++, class members are private by default, while struct members are public by default.

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Overriding redefines an inherited method in a derived class, overloading gives one function or operator several definitions, and a destructor releases memory/resources when an object is destroyed.

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fstreambase provides operations common to file streams, ifstream gives input, ofstream gives output, and fstream supports simultaneous input and output.

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Errors such as out-of-range index come from the program's own execution, so they are synchronous. Events beyond the program's control, such as keyboard interrupts, are asynchronous.

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p points to a and q points to p, so **q = 3 and *p = 3. b = 2 × (3 + 3) = 12.

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z = x++ + 17 = 33 + 17 = 50 and x becomes 34. y = ++z − ++x = 51 − 35 = 16. So x = 35, y = 16, z = 51.

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Both statements are true: a virtual function must be a member of a class, and it is accessed through object pointers (base-class pointers) for run-time polymorphism. So neither is incorrect.

Explanation

A JOIN combines rows from two or more tables based on a related (common) column between them.

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INSERT is a DML command (and is used as INSERT INTO, not "insert table"). CREATE, ALTER and DROP TABLE are DDL commands.

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Π is projection (choose columns), × is Cartesian product, and σ is selection (choose rows).

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Selecting tuples (rows) that satisfy a condition uses σ: σ Marks>80 (STUDENT). Π selects columns, not rows.

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Denormalization deliberately adds redundancy (merging tables or duplicating data) to cut joins and improve read performance.

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FLOAT stores numbers with decimal points, but BIGINT occupies 8 bytes, not 16. So only I is correct.

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DELETE is a DML command because it manipulates data. ALTER TABLE is DDL, and copy/paste are not SQL statements.

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Cardinality is the number of tuples (50), and degree is the number of attributes (20).

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Both statements are reversed: COUNT(*) counts all rows of the result, while COUNT(column) counts the non-NULL values in a column. So neither is correct.

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Ctrl+R right-aligns, Ctrl+L left-aligns, Ctrl+E centres and Ctrl+J justifies a paragraph.

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An Excel file is a workbook, which contains one or more worksheets.

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Ctrl + Enter inserts a manual page break in Word.

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PowerPoint lets you add pictures, charts, sound effects and tables to slides, so all three are correct.

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Ctrl + ] increases the selected text's font size by 1 point, and Ctrl + [ decreases it by 1 point.

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Ctrl + G (or F5) opens the Go To dialog in Excel, used to jump to a cell or range.

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Changes made to the Slide Master (such as a background image) apply to every slide that uses it.

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A website is a collection of related web pages under one domain.

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A user-friendly website should load fast, be accessible and look consistent across browsers. A cluttered design hurts usability.

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A static website serves fixed content that is the same on every load. Content changing at runtime is a feature of dynamic websites.

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<title> must be placed inside <head>, not outside it. It sets the page title shown in the browser tab, and search engines use it.

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The tag described here seems to be <meta> (the tag name is missing in the source). It holds information about the page, such as description, keywords and author.

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HTML puts no limit on the number of <form> elements a page can have (they just cannot be nested).

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An image map is one image with clickable regions (<map>/<area>), where each region links to a different URL.

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WAV is an audio file format. GIF, JPG and PNG are image formats.

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A frame buffer with one bit per pixel is a bitmap, and one with multiple bits per pixel (for colour or grey levels) is a pixmap.

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Aspect ratio is width : height = 3 : 2. With a height of 4 inches, width = 4 × 3/2 = 6 inches.

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Resolution = pixels ÷ inches = 512 / 2 = 256 pixels per inch.

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The frame buffer (refresh buffer) is the memory that stores the intensity/colour value of every pixel of the screen image.

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Aspect ratio is width to height, e.g., 16 : 9.

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Common topologies are bus, star, ring, mesh, tree and hybrid. "Singleton network" is not a topology.

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The session layer establishes, maintains and terminates sessions. Setting up the logical path (routing) between source and destination is the network layer's job, not the data link layer's.

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Half duplex is two-way but only one direction at a time, not "multidirectional". Full duplex is bidirectional at the same time. So only II is true.

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Circuit switching sets up a dedicated path before transfer, and packet switching breaks the data into packets that are routed independently. Both are true.

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Each of the n nodes connects to the other n − 1. Each edge is then counted twice, so there are n(n − 1)/2 edges.

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Quicksort partitions the array around a pivot and recursively sorts the two parts, which is divide and conquer.

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In a B-tree all leaves are at the same level, so statement (C) is false. The others are standard B-tree properties.

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By the handshaking lemma the sum of degrees is 2|E|, which is even, so the number of odd-degree vertices must be even.

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Shortest path is solvable in polynomial time (e.g., Dijkstra), so it is in P. Clique, Hamiltonian cycle and TSP (decision version) are NP-complete.

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In a red-black tree every node is red or black, and a red node's children must be black. Leaves (NIL nodes) are black, not red, so II is false.

Explanation

Add 20 at position 9 as the child of 6, then swap it with 6. Its parent is now 17, so swap again. The parent 23 is larger, so it stops: 23, 20, 14, 17, 13, 10, 1, 5, 6.

Explanation

Kruskal: take Q–R (5), S–T (10), Q–T (15); skip Q–S (20) and R–T (25), which would form cycles; take P–Q (30). Weight = 5 + 10 + 15 + 30 = 60.

Explanation

LISP stands for LISt Processing. Lists are its basic data structure.

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FOPL is First Order Predicate Logic, which uses predicates, variables and quantifiers (∀, ∃) to represent knowledge.

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In expert systems, the intelligent (knowledge-acquisition) editor is the interface the domain expert or knowledge engineer uses to enter and refine the knowledge base.

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"Any two numbers" means for all x and for all y, and "can be added" means there exists a sum z: (∀x)(∀y)(∃z)(x + y = z).

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Pattern recognition first acquires data, then extracts features, then matches them against stored patterns, and finally classifies.

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Spoofing is disguising a communication from an unknown source (e.g., a forged email address or IP) to look as if it comes from a trusted source.

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Sandboxing runs untrusted programs in a restricted, controlled environment so they cannot harm the rest of the system.

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A patch is a software update that fixes bugs or security holes, so software updates are also called patching.

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A honeypot is a decoy system set up to lure attackers so their methods can be observed and studied.

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A database includes its schema, data, relationships and constraints. A frontend is the user interface application, not part of the database.

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ROUND() is a numeric function and LENGTH() is a character function. INSERT() (e.g., in MySQL) is a string function, not a numeric one.

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A self join joins a table to itself, usually using aliases, e.g., to pair employees with their managers in the same table.

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A quaternary relationship involves 4 entity sets (binary is 2, ternary is 3).

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ACID stands for Atomicity, Consistency, Isolation and Durability. "Instance" is not one of them.

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GROUP BY puts rows with the same values in the given column(s) into groups, usually for aggregates like SUM or COUNT.

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Database models include hierarchical, network, relational and object-oriented. There is no "inheritance model".

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A data dictionary is a metadata repository that holds definitions, rules and descriptions of the data. It is not a memory area for storing data.

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SSL stands for Secure Sockets Layer, the protocol (now replaced by TLS) that encrypts traffic between a browser and a server.

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CRM covers the methods, technology and capabilities a business uses to manage and analyse its interactions with customers.

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Ciphertext is plaintext transformed by an encryption algorithm. Kerckhoffs's principle says the algorithm should be public and only the key secret, so II is false.

Explanation

Block cipher modes include ECB, CBC, CFB, OFB and CTR. "Stream (Stem) cipher mode" is not one of them.

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In distributed computing, the components of a software system are shared among multiple networked computers that work together.

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LANs, mobile networks and the Internet are distributed systems of separate networked computers. Parallel computing usually uses processors sharing memory in one machine, so it is counted as different.

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Video games, flight simulators and logic-gate simulations all model real systems. A computer table is just furniture.

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Simulation imitates how a real-world process or system behaves over time, using a model.

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In neural network theory a biological neuron is a cell that receives electrochemical signals from many sources (through dendrites) and transmits electrical impulses to other neurons.

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An axon is the long cylindrical fibre that carries impulses away from a neuron to other neurons.

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Distributed systems are loosely coupled because each node has its own memory and they communicate over a network. Parallel (multiprocessor) systems are tightly coupled because they share memory and a clock.