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