Computer Science Previous Year Questions (page 12)

Explanation

The query takes the Cartesian product R₁ × R₂ and projects A, B with duplicates removed. If R₂ is empty the product is empty, so R₂ must be non-empty; and since DISTINCT removes duplicates, the result equals R₁ only if R₁ has no duplicates.

Explanation

A → B is preserved in R1, B → C in R2, and B → D, D → B in R3; C → D follows from C → B (in R2, since C → D → B) and B → D. So all dependencies are preserved. R1 ∩ R2 = B and R2 ∩ R3 = B, and B is a key of R2 and R3, so the decomposition is also lossless.

Explanation

A relation is in 2NF if it is in 1NF and every non-key attribute is fully functionally dependent on the primary key. If every non-key attribute is functionally dependent on the primary key (no partial dependency), the relation is in second normal form.

Explanation

A⁺ contains all attributes except D, and D cannot be derived from any attribute, so D must be part of every key (e.g. AD, BD, ED, FD are keys). Then A → BC is a partial dependency of non-key attribute C on part of key AD, so R is in 1NF but not in 2NF.

Explanation

As keyed by the book, distributed systems use fixed routing; virtual and dynamic routing are not the technique keyed here.

Explanation

Quadrature amplitude modulation means changing both amplitude and phase of the Carrier.

Explanation

50,000 characters × 8 bits = 400,000 bits sent in 40 seconds, so the data rate = 400,000/40 = 10,000 bits/sec = 10 kbps.

Explanation

The three aspects of quantization programmers are concerned with are: (1) sampling rate - the number of samples per second; (2) aperture time - the time over which each sample is taken; (3) coding error - the difference between the conditioned signal and the quantized signal.

Explanation

Data link layer - framing (iv); Network layer - routing (iii); Transport layer - connection control (ii); Presentation layer - encryption (i).

Explanation

Class B has 16 host bits. Using 6 bits for the subnet leaves 10 host bits. Number of subnets = 2⁶ − 2 = 62; hosts per subnet = 2¹⁰ − 2 = 1022.

Explanation

In IPv6 the fragmentation fields of the IPv4 base header have moved to the fragmentation extension header, the authentication extension header is new in IPv6, and the record route option is not implemented in IPv6. So all the given statements are correct.

Explanation

Statistical software quality assurance in software engineering involves tracing each defect to its underlying cause, isolating the vital few causes, and moving to correct them.

Explanation

Availability = MTBF / (MTBF + MTTR). MTBF = 91.25 × 24 × 60 = 131,400 minutes and MTTR = 5 minutes, so availability = 131,400 / 131,405 = 99.9962%.

Explanation

S1 and S2 interchange white-box and black-box testing: black-box tests are based on specifications and white-box tests are based on code, so both are false. S3 correctly describes alpha testing. So only S1 and S2 are FALSE.

Explanation

Components of a memory tube (direct-view storage tube) display include the flooding gun, primary (writing) gun, collector, storage grid, screen, ground (base) and focusing and deflection system. Liquid crystal is not a component of a memory tube display.

Explanation

In oblique projection, the parallel projection rays are not perpendicular to the viewing plane but strike it at an angle other than 90°; parallel lines in space appear parallel on the projected image; and it is used for pictorial purposes rather than formal working drawings. Projectors perpendicular to the plane of projection describe orthographic projection, so (d) is not true.

Explanation

In computer graphics, horizontal retrace is the path the electron beam takes when returning to the left side of the CRT after completing a scan line.

Explanation

Context-free languages are closed under union, concatenation and Kleene star, but not under complementation (or intersection).

Explanation

The ambiguity problem for context-free grammars is undecidable: no Turing machine can always halt and decide whether an arbitrary CFG is ambiguous. Equivalence of finite automata, CFG membership and finiteness for finite automata are decidable.