Your company has a single Active Directory domain. The company network is protected by a firewall.Remote users connect to your network through a VPN server by using PPTP.When the users try to connect to the VPN server, they receive the following error message :Error 721:The remote computer is not responding.You need to ensure that users can establish a VPN connection.What should you do？（）

• A、Open port 1423 on the firewall.
• B、Open port 1723 on the firewall.
• C、Open port 3389 on the firewall.
• D、Open port 6000 on the firewall.