Your network contains two Exchange Server 2010 Client Access servers named CAS1 and CAS2 and two Mailbox servers named MBX1 and MBX2.You have one mailbox database and one public folder database.You need to ensure that the public folders are available if MBX1 fails.What should you do？（）

A. Create a Failover Cluster and then add MBX1 and MBX2 to the Failover Cluster
B. Create a database availability group （DAG） and then add MBX1 and MBX2 to the DAG
Create a public folder database on MBX2 and then modify the replication settings on each public folder
D. Create a public folder database on MBX2 and then modify the replication settings on the public folder database