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
