以下是配置PPP认证时通过debug ppp authentication得到的信息，请问该协议是几路握手?() %UPDOWN: Interface Serial0, changed state to upPPP Serial0: Send CHAP challenge id=2 to remotePPP Serial0: CHAP response id=2 received from user1PPP Serial0: Send CHAP success id=2 to remotePPP Serial0: remote passed CHAP authentication.%UPDOWN: Line protocol on Interface Serial0, changed state to up
（A）1
（B）2
（C）3
（D）4
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