Which three statements are correct regarding indexes? (Choose three.) 

Answer options:

A. A non-deferrable PRIMARY KEY or UNIQUE KEY constraint in a table automatically attempts to create a unique index.
B. Indexes should be created on columns that are frequently referenced as part of any expression.
C. When a table is dropped, corresponding indexes are automatically dropped.
D. For each DML operation performed on a table, the corresponding indexes are automatically updated if required.

ACD

References: http://viralpatel.net/blogs/understanding-primary-keypk-constraint-in-oracle/