【单项选择题】
Where is the MPLS label imposed？（）
A、A.after the Layer 3 header and before the Layer 2 payload
B、B.after the Layer 3 header and before the Layer 4 payload
C、C.after the Layer 2 header and before the Layer 1 payload
D、D.after the Layer 1 header and before the Layer 2 payload
E、E.after the Layer 2 header and before the Layer 3 payload
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