For what two purposes does the Ethernet protocol use physical addresses?（）

• A、to uniquely identify devices at Layer 2
• B、to allow communication with devices on a different network
• C、to differentiate a Layer 2 frame from a Layer 3 packet
• D、to establish a priority system to determine which device gets to transmit first
• E、to allow communication between different devices on the same network
• F、to allow detection of a remote device when its physical address is unknown