Which two statements are true about constraints?

Which two statements are true about constraints? (Choose two.)

A.
A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.

B.
The NOT NULL constraint ensures that null values are not permitted for the column.

C.
The UNIQUE constraint does not permit a null value for the column.

D.
The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.

Explanation:

:
A: A unique constraint can contain null values because null values cannot be compared to
anything.
B: The NOT NULL constraint ensure that null value are not permitted for the column
Incorrect answer :
C statement is not true
D statement is not true
Refer : Introduction to Oracle9i : SQL, Oracle University Study Guide, 10-9