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.