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An instance of a relational scheme $R(A, B, C)$ has distinct values for attribute $A$. Can you conclude that $A$ is a candidate key for $R$?

No.

 A B C $1$ $5$ $6$ $2$ $4$ $7$ $3$ $4$ $5$

Suppose this is the relational instance at any point of time.

Now we may see that $A->BC$ holds for this instance, hence A+={ABC}.

But FD s are defined on the schema itself not the instance, so based on the state of the instance we cannot say what holds for schema (there can be a many instances for R).

edited
instance of a relation is just a snapshot at any instant of time it is not whole table. it may be possible in that instance of a relation some attribute behave like key but who knows in other instance of a relation this attribute is not key, some other attribute behave like key. so w/o whole table we cannot say anything about key of rel
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