【单选题】
Which of the following is true about the MPLS header and its EXP field size？（）
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.
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