You have an Exchange Server 2010 organization named contoso.com.You have a Client Access server named cas1.contoso.com that is accessible on the Internet.You access cas1.contoso.com on the Internet by using the following domain names： （1）mail.contoso.com （2）autodiscover.contoso.com You need to ensure that mobile devices can use Auto discover on the Internet.What should you do？（）

A. Set the external URL for Microsoft-Server-ActiveSync to $NUL
B. Set the external URL for Microsoft-Server-ActiveSync to https：// cas1.contoso.com/Microsoft Server ActiveSync .
C. Set the external URL for Microsoft-Server-ActiveSync to https：//mail.contoso.com /Microsoft- Server- ActiveSync.
D. Set the external URL for Microsoft-Server-ActiveSync to http：//autodiscover.contoso.com /Microsoft- Server- ActiveSync.