You work as the enterprise exchange administrator at TestKing.com. TestKing.com makes use of Microsoft Exchange Server messaging solution. The TestKing.com network consists of a single ActiveDirectory domain named testking.com. TestKing.com has its headquarters in Chicago and a branch office in Dallas. Both offices are an ActiveDirectory site that is connected via high speed WAN link. The employees in these offices connect locallyto the servers in these offices. You are planning to install Exchange Server 2010； however， you need to plan the installation of theMailbox servers with the following criteria： * It should maintain user access to mailboxes from a server in their site， if a Mailbox server fails. * Remote mailboxes should bei accessible if a site fails.* Use the minimum amount of serves. What should you do？（）

A. The best option is to install six Mailbox servers.
B. The best option is to install four Mailbox servers.
C. The best option is to install three Mailbox servers.
D. The best option is to install two Mailbox servers.