Which of the following statements best reflects a comprehensive layered design issue in computer networks?

1. AEach layer must handle key issues like addressing, error control, flow control, sequencing and data direction, ensuring reliable communication between entities without breaking layer abstraction.
2. BThe design issues of addressing and error control are handled exclusively by the network layer, while flow control and sequencing are managed solely by the transport layer.
3. CLayered design primarily concerns selecting the physical transmission medium and ensuring minimal propagation delay across all layers.
4. DOnce error control and sequencing are implemented at the data link layer, upper layers are exempt from such design considerations.