An organization has Exchange server 2010.Network contains internal root Certification Authorization （CA）.Users on network use Outlook Anywhere.A CAS server uses a wildcard certificate issued by a trusted third party root CA.You need to ensure that users can send and receive encrypted e-mail messages by using S/MIME.What should you do？（）

A. Instruct all users to import the 3d-party root CA certification
B. Configure CAS server to use a certificate issued by a third-party CA
C. Instruct all users to import an internal root CA certificate
Deploy Outlook Web Access with the S/MIME control to the client system