Your company has a main office and 10 branch offices. Each office connects to the Internet by using a direct link.  The main office connects to the branch offices by using a WAN link. You plan to deploy Exchange Server 2010 servers in each office.  You need to design Active Directory to meet the following Exchange Server 2010 requirements：.Users must be able to access their mailboxes if a single domain controller fails .Users must be able to send e-mail messages to the Internet if a WAN link fails  What should you include in the design？（）

• A、Create an Active Directory site for each office. Deploy two global catalog servers in each site.
• B、Create an Active Directory site for each office. Deploy a single domain controller in each site，and then enable site link bridging.
• C、Create an Active Directory site for all of the offices. Deploy a global catalog server and a read-only  domain controller in each site.
• D、Create an Active Directory site for all of the offices. Deploy a global catalog server and two read-  only global catalog servers in each Active Directory site.