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.