You have a VPN server that runs Windows Server 2003 Service Pack 2 （SP2）. The VPN server supports only PPTP connections. A firewall protects the VPN server from the Internet. From a remote computer connected to the Internet， you attempt to connect to the VPN server. During the connection attempt， you receive a message that your user name and password are being verified. You then receive the following error message： “Error 721： The remote computer is not responding.”You need to ensure that you can establish a VPN connection to the VPN server from the Internet. What should you do？（）

A. On the VPN server， enable LAN routing.
B. On the VPN server， create a demand-dial interface.
C. On the firewall， allow inbound connections that use TCP port 1701.
D. On the firewall， allow inbound connections that use Generic Route Encapsulation （GRE） protocol 47.