When troubleshooting a serial link, what is the likely problem layer if an interface has an up/down status?

A) Layer 4
B) Layer 3
C) Layer 1
D) Layer 2

D
Explanation: D) When troubleshooting serial links, the following are the basic rules:
If the interface status is down/down, look to the physical layer.
If the interface status is up/down, look to the data link layer.
If the interface status is up/up, look to the network layer.