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

1. $A$ is a candidate key
2. $A$ is not a candidate key
3. $A$ is a primary key
4. Both “$A$ is a candidate key” and “$A$ is a primary key”

It is candidate key, because it has distinct value but it cant be primary key as primary key doesnt allow null value

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