Every time attribute A appears, it is matched with the same value of attribute B, but not the same value of attribute C. Therefore, it is true that:单项选择题
A
(B, C) --> A
B
A --> B
C
A --> C
D
A --> (B, C)

Question

Every time attribute A appears, it is matched with the same value of attribute B, but not the same value of attribute C. Therefore, it is true that:

BlackTom AI · Accepted Answer

The statement describes a functional dependency pattern between attributes: whenever A occurs, it is paired with the same B value (A -> B), while the C value can vary.
Option 1: '(B, C) --> A' would mean that the pair (B, C) determines A. The given rule only asserts A determines B and allows C to vary with A; it does not claim that knowing B and C together uniquely identifies A. Therefore this option is not supported by the description.
Option 2: 'A --> B' matches the described behavior: each time A appears, B has the same value, establishing a functional dependency from A to B. This is precisely what the statement conveys, so this option aligns with the given rule.
Option 3: 'A --> C' would claim that A uniquely determines C. The original sentence explicitly states that C does not have a fixed value with A (it can be different), so asserting A -> C contradicts the provided information.
Option 4: 'A --> (B, C)' would mean A determines both B and C together. Since C can vary with A according to the description, A does not determine a single value of C; thus this broader dependency is not correct given the stated variability of C.
Taken together, the only dependency that faithfully represents the described relationship is the direct A --> B option, while the others imply fixed or combined determinations that the statement does not support.

Every time attribute A appears, it is matched with the same value of attribute B, but not the same value of attribute C. Therefore, it is true that:单项选择题

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