VLAN Tagging， also known as Frame Tagging， is a method developed by Cisco to help identify packets travelling through trunk links. When an Ethernet frame traverses a trunk link， a special VLAN tag is added to the frame and sent across the trunk link .How does ERS use the VLAN tag？（）

A. provide service internetworking
B. support transparency for Layer 2 frames
C. indicate destination as a connection identifier
D. map to the DLCI in service internetworking