In an MPLS network, when a transit device receives an MPLS message and prepares to perform label exchange, it finds that the value of the label after exchange is 0. Which of the following operations will it perform at this time?

In an MPLS network, when a transit device receives an MPLS message and prepares to perform label exchange, it finds that the value of the label after exchange is 0. Which of the following operations will it perform at this time?
A . Push label 0 into the outer label of the MPLS packet and forward the MPLS packet carrying two layers of labels to the next hop
B . Exchange label 0 with the original label in the MPLS message and forward the message to the next hop
C . Pop the label in the MPLS message and forward the message to the next hop
D . Do not process the label and directly forward the message to the next hop

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Answer: B