In the OSI model, at which layers do WANs operate at?（）

A. Data Link
B. Session
C. Physical
D. Presentation
E. Network
F. Transport

A. It encourages industry standardization by defining what functions occur at each layer of the model.
B. It necessitates changes in functionality in one layer to other layers.
C. It enables equipment efficiency from different vendors to use the same electronic components.
D. It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
E. It supports the evolution of multiple competing standards, and thus enhances business equipment manufacturing opportunities.

A.RIP
B.STP
C.IGRP
D.CDP
E.VTP