，Kerberos三步认证过程是通过三个子协议来完成的。这三个子协议分别是(____)。
A.Authentication Service Exchange、Ticket GrantingService Exchange和Client/Server Exchange
B.Authentication Service Exchange、SSH 、SMTP
C.Ticket GrantingService Exchange、SMTP、NTP
D.Client/Server Exchange、NTP、SSH
正确答案是A