Your network contains a single Active Directory domain. The functional level of the forest is Windows  Server 2008. The functional level of the domain is Windows Server 2008 R2.   All DNS servers run Windows Server 2008. All domain controllers run Windows Server 2008 R2.  You need to ensure that you can enable the Active Directory Recycle Bin.   What should you do（）

• A、Change the functional level of the forest.
• B、Change the functional level of the domain.
• C、Modify the Active Directory schema.
• D、Modify the Universal Group Membership Caching settings.