What is the cause of the problem?

Refer to the exhibit. The Bigtime router is unable to authenticate to the Littletime router. What is the cause of the problem?

A.
The usernames are incorrectly configured on the two routers.

B.
The passwords do not match on the two routers.

C.
CHAP authentication cannot be used on a serial interface.

D.
The routers cannot be connected from interface S0/0 to interface S0/0.

E.
With CHAP authentication, one router must authenticate to another router. The routers cannot be configured to authenticate to each other.