Which two statements about using the CHAP authentication mechanism in a PPP link are true?（）

• A、CHAP uses a two-way handshake
• B、CHAP authentication periodically occurs after link establishment
• C、CHAP has no protection from playback attacks
• D、CHAP authentication is performed only upon link establishment
• E、CHAP uses a three-way handshake
• F、CHAP authentication passwords are sent in plaintext