You work as the exchange administrator at TestKing.com. The TestKing.com network contains an Exchange Server 2010 Organization. You are responsible for managing the Exchange network for TestKing.com The organization contains three Exchange Server 2010 Edge Transport Servers and seven ExchangeServer 2010 Hub Transport servers. The Edge Transport server is used to transfer e-mail from your organization to the Internet. During the course of day you receive an instruction form the CIO to ensure that all clients is able to send encrypted messages to other organizations over the Internet as well asensuring that communications between the Hub Transport servers and the Edge Transport servers isencrypted. What should you do？（）

A. Your best option would be to deploy SMTP over SS
B. Your best option would be to implement Security/Multipurpose Internet Mail Extensions （S/MINE）.
C. Your best option would be to deploy IPsec.
D. Your best option would be to implement Domain Securtiy.