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

When using a dual slot line group MSP ring, in which of the following cases is the multiplex section overhead K bytes in the pass-through state?
A . Cross-connect board abnormality
B . Main control board abnormality
C . The main control board is not in place
D . When the multiplex section protocol controller state is pass-through

Answer: C