Which four of these are MPLS label header fields？ （）

A.SNMP traps
B.fabric cards
C.exec-level configuration
D.privilege-level configuration
E.admin-level configuration

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