which three situations must you collect optimizer statistics manually for database objects in addition to automatic statistics collection?

In which three situations must you collect optimizer statistics manually for database objects in

addition to automatic statistics collection?

A.
When substantial DML activity occurs between the nightly automatic stats gathering
maintenance job

B.
When substantial activity occurs on a partition of the partitioned table.

C.
When a table is used for bulk loads that add 10% or more to the total size of the table

D.
When an index is created or dropped for a column

E.
When the degree of parallelism is explicitly defined for a table

Explanation:
When to Gather Statistics
When gathering statistics manually, you not only need to determine how to gather statistics, but
also when and how often to gather new statistics.
For an application in which tables are being incrementally modified, you may only need to gather
new statistics every week or every month. The simplest way to gather statistics in these
environment is to use a script or job scheduling tool to regularly run the
GATHER_SCHEMA_STATS and GATHER_DATABASE_STATS procedures. The frequency of
collection intervals should balance the task of providing accurate statistics for the optimizer against
the processing overhead incurred by the statistics collection process.
(C) For tables which are being substantially modified in batch operations, such as with bulk loads,
statistics should be gathered on those tables as part of the batch operation. The DBMS_STATS
procedure should be called as soon as the load operation completes.
For partitioned tables, there are often cases in which only a single partition is modified. In those
cases, statistics can be gathered only on those partitions rather than gathering statistics for the
entire table. However, gathering global statistics for the partitioned table may still be necessary.