You have an Exchange Server 2010 organization that contains a single Mailbox server.The Mailbox server contains two mailbox databases named DB1 and DB2.You need to restore a backup of DB1.Users must be able to connect to their mailboxes on DB1 during the restore process.What should you create first？（）
- Aa database availability group （DAG）
- Ba Client Access server array
- Ca Recovery Mailbox database
- Da recovery storage grou