After you configure an access switch on your network, you receive a call that the users have lost connectivity. You examine the trunk interface with the command show interfaces Gi 0/1. What might be the problem?

A. A spanning-tree loop has been detected.
B. The switch uplink cable is bad.
C. BPDU Guard was configured on a trunk.
D. Flow control is turned off.

C. BPDU Guard was turned on the trunk link. When the BPDU of the adjacent switch was seen, the switch turned the port into err-disabled mode.