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.