What is the action of “pop” in the context of MPLS switching?

A.
It replaces the top label in the MPLS label stack with a set of labels.

B.
It replaces the top label in the MPLS label stack with another value.

C.
None of above.

D.
It removes the top label in the MPLS label stack.

E.
It adds a top label in MPLS label stack.