You have Exchange Server 2003 organization.  The organization contains a front-end server named FE1 and a back-end server named BE1. FE1 is accessible from the Internet by using mail.contoso.com. You plan to transition the organization to Exchange Server 2010.  You will deploy a Mailbox server named MBX1 and a Client Access server named CAS1. Users will access Outlook Web App （OWA） by using the URL https：//mail.contoso.com. You need to recommend a DNS configuration for the external name of mail.contoso.com.  Which server should be associated with the name mail.contoso.com？（）

• A、BE1
• B、CAS1
• C、FE1
• D、MBX1