You plan to install Exchange Server 2010 Service Pack 1 （SP1） servers in an existing Exchange Server 2007 organization.All servers will coexist in a single Active Directory site. You need to identify which role must be transitioned to Exchange Server 2010 first. Which role should you identify？（）

• A、Hub Transport.
• B、Mailbox Server.
• C、Unfied Messaging.
• D、Client Access Server.