Dependency inference: Every time attribute A appears it has the same B value, but C varies. What dependency statement is therefore true?

Introduction / Context:
Functional dependencies are derived from observed or intended business rules. This question asks you to infer a dependency from observed consistency.

Given Data / Assumptions:

• Whenever A has a certain value, B always takes the same value with it.
• C does not remain the same for a given A; it can vary.

Concept / Approach:
If B is uniquely determined by A, we express this as A determines B. Since C can vary for the same A, A does not determine C, and therefore A does not determine the pair (B, C) either.

Step-by-Step Solution:

Verification / Alternative check:
Construct a small sample with fixed pairs of A and B but multiple C values; you will see that B is functionally dependent on A while C is not.

Why Other Options Are Wrong:

• A → C contradicts the stated variation of C for a given A.
• A → (B, C) fails because A cannot determine both when C is not fixed.
• (B, C) → A is not implied; it would require each pair of B and C to map to exactly one A.

Common Pitfalls:
Assuming that if A determines one attribute, it determines all related attributes. Each dependency must be justified independently.

Final Answer:
A → B.