There is a connectivity problem between the serial 0/0 interface of router TESTKING1 and the serial 0/0 interface of TESTKING2. A leased line point to point circuit is installed between these two routers. The output from the "show interface serial 0/0" command is displayed below for each of these routers: From your observations of the above exhibit, what is the underlying cause of the problem?（）

• A、The loopback is not set.
• B、The serial cable is faulty.
• C、The subnet mask is not configured properly.
• D、The IP address is not configured properly.
• E、The Layer 2 frame types are not compatible.
• F、The keepalive setting is not configured properly.