Your company has two main offices named Main1 and Main2.An Active Directory site exists for each office. Users connect locally to servers in both offices. The offices connect to each other by using a high-speed WAN link. You plan to deploy Exchange Server 2010. You need to plan the deployment of Mailbox servers to meet the following requirements： .Ensure that users can access their mailbox from a server in their site， if a single Mailbox server fails .Ensure that users can access their mailboxes remotely if a site fails .Minimize the number of servers How many Mailbox servers should you include in the plan？（）