An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true?

An instance of relational schema R (A, B, C) has distinct values of A

1.	An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true?
A.	A is a candidate key
B.	A is not a candidate key
C.	A is a primary Key
D.	Both (A) and (C)
Answer» C. A is a primary Key