Which command is used to enable CHAP authentication, with PAP as the fallback method, on a serial interface?
CHAP authentication
A. Router(config-if)# ppp authentication chap fallback ppp
B. Router(config-if)# ppp authentication chap pap
C. Router(config-if)# authentication ppp chap fallback ppp
D. Router(config-if)# authentication ppp chap pap