When you enter the interface command auto qos voip cisco-phone on a port at the edge of a network,how does the Cisco Catalyst switch detect if a Cisco IP Phone is connected to the port?（）

• A、by snooping the CoS marking on the incoming frames
• B、by using RTP hello messages between the switch port and the Cisco IP Phone
• C、by using CDP
• D、by using DHCP snooping
• E、by snooping the incoming 802.1Q VLAN tag