which scenario would an index be most useful?

In which scenario would an index be most useful?

A.
The indexed columns are part of an expression.

B.
The indexed column is declared as NOT NULL.

C.
The indexed column contains a wide range of values.

D.
The indexed columns are used in the FROM clause.

Explanation:

Index will be useful if the indexed column contains a wide range of values. Especially B-tree
indexes will work better for tables with a wide range of values. But for tables just with some distinct
values bitmap indexes will be more helpful.