You have an Exchange organization that contains Exchange 2000 Server Service Pack 3 （SP3）， Exchange Server 2003 Service Pack 2 （SP2）， and Exchange Server 2007 Service Pack 1 （SP1） servers. You need to transition the organization to Exchange Server 2010.  What should you do first？（）

• A、Remove all Exchange Server 2007 SP1 servers from the organization.
• B、Remove all Exchange 2000 Server and all Exchange Server 2003 servers from the organization.
• C、Remove all Exchange 2000 Server servers from the organization， and then upgrade all Exchange Server 2007 servers to SP2.
• D、Remove all Exchange Server 2003 servers from the organization， and then upgrade all Exchange Server 2007 servers to SP2.