Which two statements regarding external authentication servers for firewall userauthentication are true？（）

• A、Up to three external authentication server types can be used simultaneously.
• B、Only one external authentication server type can be used simultaneously.
• C、If the local password database is not configured in the authentication order， and the configured authentication server is unreachable， authentication is not performed.
• D、If the local password database is not configured in the authentication order， and the configured authentication server rejects the authentication request， authentication is not performed