Your company has a main office and two branch offices. The main office is located in Montreal. The branch offices are located in Seattle and Winnipeg. In the Montreal office， you have a server named Server1 that runs Windows Server 2003 Web Edition Service Pack 2 （SP2）. Server1 is configured as a VPN router. In the Seattle office， you have a VPN server named Server2. In the Winnipeg office， you have a VPN server named Server3. On Server1， you configure a L2TP/IPSec demand-dial interface for each branch office. On each branch office server， you create a single demand-dial interface that uses L2TP/IPSec to connect to the main office. All demand-dial interfaces are configured to use a pre-shared key. You discover that only one branch office can connect to the main office at a time. You need to ensure that both branch offices can connect to the main office simultaneously.  What should you do？（）

• A、Add two more demand-dial interfaces on Server1.
• B、Install a computer certificate on Server1， Server2， and Server3.
• C、Upgrade Server1 to Windows Server 2003 Standard Edition SP2.
• D、Add an additional demand-dial interface on Server2 and Server3.