Which three statements about classification marking of traffic at Layer 2 are true？ （）

A.a Frame Relay header includes a 1-bit discard eligible （DE） bit to provide the class of service （CoS）

B.an ATM header includes a 1-bit DE field to provide the CoS

C.an MPLS EXP field is inserted in the Layer 3 IP precedence field to identify the CoS

D.the CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link （ISL） trunking is used

E.in the IEEE 802.1p standard， three bits are used to identify the user priority bits for the CoS

F.in the IEEE 802.1q standard， six bits are used to identify the user priority bits for the CoS