You have an Exchange organization.All servers in the organization have Exchange Server 2010 Service Pack 1 （SP1） installed.The network contains an internal root certification authority （CA）.Users on the network use Outlook Anywhere.A Client Access 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 third-party root CA certificate.
B. Import the internal root CA certificate to the Client Access server.
C. Instruct all users to import the internal root CA certificate.
D. Issue a certificate to each user from the internal root CA