Which two statements are true about multiple Exchange Server versions in the same e-mail domain?

Which two statements are true about multiple Exchange Server versions in the same e-mail domain? (Choose two.)
A. All of the Exchange Servers need to be updated to the same version of Windows Server.
B. An administrator needs to install separate instances of Email Notification Services (ENS).
C. The ENS server is able to complete all the subscriptions correctly.
D. The ENS server functionality is interrupted.
E. Each Exchange server needs to have at least one Apple Push Notification Service (APNS) Certificate.