When should the "clock rate" command be used on a Cisco router’s interface?

When should the “clock rate” command be used on a Cisco router’s interface?

A.
During diagnostic functions such as a loopback test

B.
When the router is directly connected to another Cisco router

C.
When the router is connected to a Frame Relay switch

D.
When the interface is connected to a CSU/DSU

Explanation: