You have an Exchange Server 2010 organization.Users on the network use HTTPS to connect to Outlook Web App （OWA）.The Client Access server uses a certificate issued by an internal certification authority （CA）.You plan to deploy an e-mail encryption solution for all users. You need to ensure that users can send and receive encrypted e-mail messages by using S/MIME from OWA.What should you do？（）

• A、Issue a certificate to each user
• B、Instruct all OWA users to import a root CA certificate
• C、Modify the authentication settings of the OWA virtual directory
• D、Configure the Client Access Server to use a certificate issued by a third-party CA