The Testking network is implementing dialup services for their remote employees.Testking uses several different Layer 3 protocols on the network. Authentication ofthe users connecting to the network is required for security. Additionally, some employees will be dialing long distance and will need callback support.Which protocol is the best choice for these remote access services?（）

• A、802.1
• B、Framerelay
• C、HDLC
• D、PPP
• E、SLIP
• F、PAP