Your network consists of a single Active Directory domain. The functional level of the domain is Windows Server 2003. You install an application that requires a user account enabled for constrained delegation. You create the required user account. When you attempt to configure constrained delegation, you discover that the Delegation tab does not appear in the user account properties. You need to ensure that the Delegation tab appears in the user account properties. What should you do?（）

A. Raise the forest functional level.
B. Add the user account to the Domain Admins group.
C. Register a service principal name (SPN) for the user account.
D. Select the Trust this computer for delegation to specified services only option on all domain controllers.