Your company has three offices. Each office has 1,000 users and is configured as an Active Directory site.
Each site connects directly to the Internet.
The network contains six servers that have Exchange Server 2013 installed. The servers are configured as
shown in the following table.

An administrator performs a datacenter switchover by changing the DNS record for webl.adatum.com to point
to the IP address of web2.adatum.com.
Users from Main1 report that when they connect to Outlook Web App, they receive a certificate warning
message.
You need to recommend a solution to ensure that the users do not receive a certificate warning message whena datacenter switchover is performed.
What should you include in the recommendation?

A.
Three host headers for the Default Web Site on each Client Access server

B.
One certificate that contains all of the external names as subject alternative names

C.
Three certificates that each contains one of the external names

D.
An external URL for each Client Access server set to $null

Explanation:
SAN Certificates (Subject Alternative Names)
This type of certificate allows more than a single name in a single SSL certificate which makes total sense for
the new Microsoft products (Lync and Exchange) because several services are using names and all of them
are underneath the same IIS Web Site. In some Public CA these certificates are also known as UC Certificates.
Managing Certificates in Exchange Server 2013