Your company has a main office and 50 branch offices. Each office is configured as an Active Directory site. Each branch office site contains a domain controller.  The main office site contains all the global catalog servers in the forest. Each branch office contains a WAN link that connects to the main office.  You need to plan the deployment of new Mailbox servers to meet the following requirements： .Ensure that users in the branch offices can access their mailboxes if their local domain controller fails .Deploy the minimum number of Exchange servers What should you include in the plan？（）