For what two purposes does the Ethernet protocol use physical addresses？（）
- Ato uniquely identify devices at Layer 2
- Bto allow communication with devices on a different network
- Cto differentiate a Layer 2 frame from a Layer 3 packet
- Dto establish a priority system to determine which device gets to transmit first
- Eto allow communication between different devices on the same network
- Fto allow detection of a remote device when its physical address is unknow