【单选题】
What is the action of "pop" in the context of MPLS switching?()
A、It removes the top label in the MPLS label stack.
B、It adds a top label in MPLS label stack.
C、It replaces the top label in the MPLS label stack with another value.
D、It replaces the top label in the MPLS label stack with a set of labels.
E、None of above.