You deploy a Windows Server 2008 VPN server behind a firewall. Remote users connect to the VPN by using portable computers that run Windows Vista with the latest service pack.  The firewall is configured to allow only secured Web communications. You need to enable remote users to connect as securely as possible. You must achieve this goal without opening any additional ports on the firewall.What should you do？（）

• A、Create an IPsec tunnel.
• B、Create an SSTP VPN connection.
• C、Create a PPTP VPN connection.
• D、Create an L2TP VPN connection.