You need to recommend a solution to ensure that users can access their mailbox on EX1 or EX4 during the migration

You have an Exchange Server 2010 organization that contains three servers. The servers are
configured as shown in the following table.

Microsoft Outlook is configured to connect to an FQDN of mail.adatum.com. Mail.adatum.com
resolves to the IP address of a Layer 7 hardware load balancer. The hardware load balancer is
configured to send traffic to EX2 and EX3.
You deploy an Exchange Server 2013 Mailbox server named EX4 and an Exchange Server 2013 Client
Access server named EX5.
You plan to migrate all mailboxes to Exchange Server 2013.
You need to recommend a solution to ensure that users can access their mailbox on EX1 or EX4
during the migration.
What should you do?

A.
Create a Client Access server array that contains EX1 and EX5.

B.
Modify the autodiscover.contoso.com resource record to point to EX5.

C.
Modify the properties of the hardware load balancer to point to EX5.

D.
Create a DNS record named legacy.contoso.com in the internal DNS zone.

Explanation:
Note: So what is the Client Access server in Exchange 2013? The Client Access server role is
comprised of three components, client protocols, SMTP, and a UM Call Router. The CAS role is a thin,
protocol session stateless server that is organized into a load balanced configuration. Unlike previous
versions, session affinity is not required at the load balancer (but you still want a load balancer to
handle connection management policies and health checking). This is because logic now exists in CAS
to authenticate the request, and then route the request to the Mailbox server that hosts the active
copy of the mailbox database.
The Mailbox server role now hosts all the components and/or protocols that process, render and
store the data. No clients will ever connect directly to the Mailbox server role; all client connections
are handled by the Client Access server role.
Reference: Exchange 2013 Server Role Architecture