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