You plan to install Exchange Server 2010 SP1 servers in an existing Exchange Server 2007 organization. All servers will co-exist 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.
Client Access.

B.
Hub Transport.

C.
Mailbox.

D.
Unified Messaging (UM).