Your network is configured as shown in the following table.

You shut down Server2 for maintenance and discover that Server4 now takes longer than normal to deliver e-mail.

You need to ensure that the performance of Server4 remains unaffected if Server2 fails.

What should you do?

A.
Configure Server3 as a global catalog server.

B.
Enable Universal Group Membership Caching for Site2.

C.
Install an additional domain controller for contoso.com in Site2.

D.
Configure Server3 as the preferred bridgehead server for Active Directory replication.