Your network consists of an Active Directory forest that contains two sites named Site1 and Site2. From the Internet， users can only access servers in Site1. You have an Exchange Server 2007 organization. Each site contains servers that run the Client Access server role， the Mailbox server role， and the HubTransport server role. You plan to transition the organization to Exchange Server 2010. You need to recommend the server role that you must transition first to Exchange Server 2010. Your solution must provide the least amount of downtime for users in the organization. Which servers should you transition first？（）

A. the Client Access servers in Site1
B. the Client Access servers in Site2
C. the Hub Transport servers in Site1
D. the Hub Transport servers in Site2