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