Refer to the exhibit. Spanning tree protocol is running on all three switches. The switches are configured so that Link A is the active link, and Link B is the standby link. There is a problem occurring where Switch B starts forwarding on Link B causing a routing loop.
What is the likely cause of the problem?
A. PortFast is not enabled.
B. There is a port duplex mismatch.
C. MISTP is enabled without RSTP.
D. A single instance of STP is enabled instead of PVST.
https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html#duplex