Which statements are correct regarding indexes?

Which statements are correct regarding indexes? (Choose all that apply.)

A.
When a table is dropped, the corresponding indexes are automatically dropped.

B.
For each DML operation performed, the corresponding indexes are automatically updated.

C.
Indexes should be created on columns that are frequently referenced as part of an expression.

D.
A non-deferrable PRIMARY KEY or UNIQUE KEY constraint in a table automatically creates a
unique index.