Your network contains one Exchange Server 2010 Service Pack 1 （SP1） Client Access server named Server1. You have a user named User1. You need to limit the size of the attachments that User1 can download by using Windows Mobile 6.5 devices. What should you do first？（）

A. Create a new Exchange ActiveSync device access rule.
B. Create new managed content settings.
C. Modify the message size restrictions for User1.
D. Create a new Exchange ActiveSync mailbox policy.