Your company has three offices. Each office has a direct link to the Internet. The offices connect to each other by using a WAN link. Your network consists of an Active Directory forest that contains two domains and one site. The functional level of the forest is Windows Server 2003. All domain controllers run Windows Server 2003 R2. Each office contains two domain controllers for eachdomain. All domain controllers are global catalog servers. In each office， you plan to deploy Mailbox， Client Access， and Hub Transport Exchange Server 2010servers. All e-mail messages sent to the Internet will be delivered from a local server in each office. You need to recommend changes to the Active Directory environment to support the planned deploymentof Exchange Server 2010. What should you recommend？（）

A. Disable site link bridging for the forest.
B. Modify the cost values for the default IP site link.
Create an Active Directory subnet and site object for each office.
D. Upgrade one domain controller in each office to Windows Server 2008.