In the context of MPLS，what is the correct action to be performed when an aggregate label is received ？（）
A. It adds the VPN label into the top label in the MPLS label stack
B. It replaces the top label in the MPLS label stack with a VPN label and propagates
C. It replaces the bottom label in the MPLS label stack with a VPN label and propagates
D. It removes the top label in the MPLS label stack and does a Layer 3 lookup on underlying IP packets
E. None of above
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