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单项选择题

A. (config-if)#authentication ppp chap fallback pppB. (……

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