Your network contains an Active Directory forest. The forest contains three domains. All
domain controllers have the DNS Server server role installed.
The forest contains three sites named Site1, Site2, and Site3. Each site contains the users,
client computers, and domain controllers of each domain. Site1 contains the first domain
controller deployed to the forest.
The sites connect to each other by using unreliable WAN links.
The users in Site2 and Site3 report that is takes a long time to log on to their client computer
when they use their user principal name (UPN). The users in Site1 do not experience the
same issue.
You need to reduce the amount of time it takes for the Site2 users and the Site3 users to log
on to their client computer by using their UPN.
What should you do?

A.
Configure a global catalog server in Site2 and a global catalog server in Site3.

B.
Reduce the replication interval of the site links.

C.
Move a primary domain controller (PDC) emulator to Site2 and to Site3.

D.
Add additional domain controllers to Site2 and to Site3.

E.
Reduce the cost of the site links.

F.
Enable universal group membership caching in Site2 and in Site3.

Explanation:
http://technet.microsoft.com/en-us/library/cc728188.aspx
Common Global Catalog Scenarios
The following events require a global catalog server:
(…) User logon. In a forest that has more than one domain, two conditions require the global
catalog during user authentication:
1. When a user principal name (UPN) is used at logon and the forest has more than one
domain, a global catalog server is required to resolve the name.
2. (…)