which command is used to enable CHAP authentication whit PAP as the fallback method on a serial interface?

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

Answer: C