Which two statements correctly describe UDLD？ （）

• A、Unidirectional link detection is achieved through monitoring STP BPDUs.
• B、It is recommended that it be enabled globally rather than on an individual port.
• C、When an error is detected， normal mode will disable a port.
• D、It is recommended that it be enabled in normal mode.
• E、It is recommended that it be used with the loop guard feature.