Where is the MPLS label imposed?

A.
after the Layer 3 header and before the Layer 2 payload

B.
after the Layer 3 header and before the Layer 4 payload

C.
after the Layer 2 header and before the Layer 1 payload

D.
after the Layer 1 header and before the Layer 2 payload

E.
after the Layer 2 header and before the Layer 3 payload