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