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

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

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