In optical channel transport unit overhead （OTU OH）， what are general communication channels 1 and 2（GCC1/GCC2） used for？（）

• A、for trail trace identification
• B、as the backward defect indicator
• C、to transmit information between OTU termination points
• D、to extend command and management functions over several frames
• E、General communication channels 1 and 2 （GCC1/GCC2） do not belong to OTU OH