Your network contains an Active Directory forest named contoso.com.
The forest contains a single domain.
The forest contains three Active Directory sites named SiteA, SiteB, and SiteC.
The sites contain four domain controllers.
The domain controllers are configured as shown in the following table.

An IP site link exits between each site.
You discover that the users in SiteC are authenticated by the domain controllers in SiteA and SiteB.
You need to ensure that the SiteC users are authenticated by the domain controllers in SiteB, unless all of the domain controllers in SiteB are unavailable.
What should you do?

A.
Create a site link bridge.

B.
Create additional connection objects for DC3 and DC4.

C.
Create additional connection objects for DC1 and DC2.

D.
Increase the cost of the site link between SiteA and SiteC.

Explanation:
http://technet.microsoft.com/en-us/library/dd277430.aspx#XSLTsection126121120120