Your network contains an Active Directory domain named contoso.com. The domain contains the  servers shown in the following table：     Server name  Operating system  Role   DC1  Windows Server 2008  Domain controller  DC2  Windows Server 2008 R2  Domain controller  DNS1  Windows Server 2008  DNS server  DNS2  Windows Server 2008 R2  DNS server     The functional level of the forest is Windows Server 2003. The functional level of the domain is  Windows Server 2003.   DNS1 and DNS2 host the contoso.com zone. All client computers run Windows 7 Enterprise.     You need to ensure that all of the names in the contoso.com zone are secured by using  DNSSEC.   What should you do first（）

• A、Change the functional level of the forest.
• B、Change the functional level of the domain.
• C、Upgrade DC1 to Windows Server 2008 R2.
• D、Upgrade DNS1 to Windows Server 2008 R2.