You have an Exchange Server 2010 organization.Your network contains two Active Directory sites named Site1 and Site2.Each site contains five Mailbox servers.A Mailbox server named Server1 is in Site1.A Mailbox server named Server2 is in Site2.You have a public folder named Folder1 that is replicated to Server1 and Server2.Users in Site1 report that they cannot access Folder1 when Server1 is offline.You need to ensure that all users can access Folder1 if Server1 is offline.What should you do？（）

• A、Add Server1 to the public folder referrals for Site2.
• B、Add Server2 to the public folder referrals for Site1.
• C、Add Server1 and Server2 to a Client Access server array.
• D、Add Server1 and Server2 to a database availability group （DAG）.