You deploy a windows server 2008 VPN server behind a firewall. Remote uses connect to the VPN by using portable computers that run windows vista with the latest service pack. The firewall is configured to allows 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.
Create a PPTP VPN connection.
D. Create a L2tp VPN connection.