What is the function of the "S bit" in the MPLS header?

A.
It is a service bit which when set indicates this label is active.

B.
It is a bottom of stack bit which when set indicated that this is not the last label in the stack.

C.
It is a bottom of stack bit which when set it indicates that this is the last label in the stack.

D.
It is a service bit which when set indicates this label is not active.