Your company has 10 servers that run Windows Server 2008 R2. The servers have Remote Desktop Protocol (RDP) enabled for server administration. RDP is configured to use default security settings. All administrators' computers run Windows 7. You need to ensure the RDP connections are as secure as possible. Which two actions should you perform？（）

A. Set the security layer for each server to the RDP Security Layer.
B. Configure the firewall on each server to block port 3389.
C. Acquire user certificates from the internal certification authority.
D. Configure each server to allow connections only to Remote Desktop client computers that use Network Level Authentication.