问题：

[单选] 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.