What might have caused three of the alerts to disappear?

Examine the query and its output:

What might have caused three of the alerts to disappear?

A.
The threshold alerts were cleared and transferred to DBA_ALERT_HISTORY.

B.
An Automatic Workload Repository (AWR) snapshot was taken before the execution of the second query.

C.
An Automatic Database Diagnostic Monitor (ADOM) report was generated before the execution of the
second query.

D.
The database instance was restarted before the execution of the second query.