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