TestKing.com is configuring the serial interface of a Cisco router to connect to the router of a new ISP. A full T1 is being used for the Internet connection. After issuing the show interface serial 0/0 command, it is observed that the interface is UP and the line protocol is DOWN. Which of the following commands could fix this problem?（）

• A、Border# copy running-config startup-config
• B、Border(config)# no shutdown
• C、Border(config-if)# encapsulation ppp
• D、Border(config-if)# no cdp enable
• E、Border(config-if)# ip routing