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.