A.The MPLS header is 2 bytes，and the EXP field is 3 bits long.
B.The MPLS header is 1 byte，and the EXP field is 3 bits long.
C.The MPLS header is 4 bytes，and the EXP field is 3 bits long.
D.The MPLS header is 3 bytes，and the EXP field is 3 bits long.