You have an Exchange Server 2010 organization.  The network contains an Exchange Server 2010 Mailbox server named Server1. All mailboxes are stored on Server1.  You perform a Typical installation of Exchange Server 2010 on a new server named Server2. You plan to implement redundancy for mailbox access.  You need to recommend a solution that ensures that client computers can reconnect to their mailboxwithin five minutes if Server1 fails.  What should you recommend？（）

• A、Configure cluster continuous replication （CCR）. Implement a file share witness.
• B、Configure a Network Load Balancing cluster that includes Server1 and Server2. Implement Active Directory-integrated DNS zones.
• C、Configure a database availability group （DAG） that includes Server1 and Server2. Set the time to  live （TTL） for the DNS record.
• D、Configure a database availability group （DAG） that includes Server1 and Server2. Use the same certificate for both servers.