Which three metrics should you monitor?

An application sends Azure push notifications to a client application that runs on Windows Phone, iOS, and
Android devices.
Users cannot use the application on some devices. The authentication mechanisms that the application uses
are the source of the problem.
You need to monitor the number of notifications that failed because of authentication errors.
Which three metrics should you monitor? Each correct answer presents part of the solution.

A.
Microsoft Push Notification Service (MPNS) authentication errors

B.
External notification system errors

C.
Apple Push Notification Service (APNS) authentication errors

D.
Channel errors

E.
Windows Push Notification Services (WNS) authentication errors

F.
Google Cloud Messaging (GCM) authentication errors