(5) [3pt] Why are NULL values needed in the relational model? NULL values can be used for all except which one of the following? A. To fill a column in tuple when that column does not really "exist for that particular tuple. B. To leave columns in a tuple marked as unknown when the actual value is unknown. C. To avoid confusion with actual legitimate data values like 0 for integer columns and (the empty string) for string columns. D. To allow duplicate tuples in the table by filling the primary key column(s) with NULL