Your network contains an Active Directory forest. All domain controllers run Windows Server 2008 Standard. The functional level of the domain is Windows Server 2003. You have a certification authority （CA）. The relevant servers in the domain are configured as shown in the following table： Server name Operating system Server role Server1 Windows Server 2003 Enterprise root CA Server2 Windows Server 2008 Enterprise subordinate CA Server3 Windows Server 2008 R2 Web Server You need to ensure that you can install the Active Directory Certificate Services （AD CS） Certificate Enrollment Web Service on the network. What should you do（）

A. Upgrade Server1 to Windows Server 2008 R2.
B. Upgrade Server2 to Windows Server 2008 R2.
C. Raise the functional level of the domain to Windows Server 2008.
D. Install the Windows Server 2008 R2 Active Directory Schema updates.