What is true of using group functions on columns that contain NULL values?

What is true of using group functions on columns that contain NULL values?

A.
Group functions on columns ignore NULL values.

B.
Group functions on columns cannot be accurately used on columns that contain NULL values.

C.
Group functions on columns include NULL values in calculations if you use the keyword
INC_NULLS.

D.
Group functions on columns returning dates include NULL values.

E.
Group functions on columns returning numbers include NULL values.

Explanation:

: group functions on column ignore NULL values
Incorrect answer :
* group functions on column ignore NULL values
* group functions on column ignore NULL values
* NVL function can be use for column with NULL values
* no such INC_NULLS keyword

Refer : Introduction to Oracle9i : SQL, Oracle University Study Guide, 5-12