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