When using the dual slot line group MSP ring, which of the following situations is the multiplex section overhead K bytes in the pass-through state?

When using the dual slot line group MSP ring, which of the following situations is the multiplex section overhead K bytes in the pass-through state?
A . The cross board is abnormal
B. The main control board is abnormal
C. The main control board is not in place
D. When the status of the multiplex section protocol controller is pass-through

Answer: C