In a relational database model, NULL values can be used for all but which one of the following?

To avoid confusion with actual legitimate data values like 0 (zero) or integer columns and " (the empty string) for string columns

To allow duplicate tuples in the table by filling the primary key column(s) with NULL

To leave columns in a tuple marked as "unknown" when the actual value is unknown

To fill a column in a tuple when that column does not really "exist" for that particular tuple

In a relational database model, NULL values can be used for all but wh

1.	In a relational database model, NULL values can be used for all but which one of the following?
A.	To allow duplicate tuples in the table by filling the primary key column(s) with NULL
B.	To avoid confusion with actual legitimate data values like 0 (zero) or integer columns and " (the empty string) for string columns
C.	To leave columns in a tuple marked as "unknown" when the actual value is unknown
D.	To fill a column in a tuple when that column does not really "exist" for that particular tuple
Answer» B. To avoid confusion with actual legitimate data values like 0 (zero) or integer columns and " (the empty string) for string columns