Your company has two data centers named Main1 and Main2. Each data center connects directly to the
Internet and has a link to the other data center. All servers run Windows Server 2012 R2.
The network contains an Active Directory forest. Each data center is configured as an Active Directory site.
You plan to deploy two Exchange Server 2013 servers to each data center. Each data center will host 500 user
mailboxes for its respective users. All of the users will connect remotely to access their mailbox.
You need to recommend a deployment solution that meets the following requirements:
The users from both data centers must be able to access their mailbox if the link between the data centers
fails.
The users from both data centers must be able to access their mailbox if a single server fails.
The solution must minimize manual intervention if a site fails.
What is the best recommendation to achieve the goal? More than one answer choice may achieve the goal.
Select the BEST answer.

A.
two database availability groups (DAGs) that each contains one server from each data center

B.
one database availability group (DAG) that contains all of the servers

C.
two database availability groups (DAGs) that each contains all of the servers

D.
one database availability group (DAG) that contains all of the servers in Main1 and one DAG that contains
all of the servers in Main2.

Explanation:
https://technet.microsoft.com/en-us/library/dd638129(v=exchg.150).aspx