Which command is used to enable CHAP authentication, with PAP as the fallback method, on a
serial interface?

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