What is the function of an implicit-null label?

What is the function of an implicit-null label?

A.
It notifies the upstream LSR to remove the top label in the label stack and forward the packet.

B.
It notifies the upstream LSR to add a VPN label to the label stack.

C.
It is used to statically assign a label to an IGP route.

D.
It is used to identify the router ID.