Relation R(A, B, C, D, E) has functional dependencies A -> B, B -> C and CD -> E. What is the attribute closure {A}+?
Correct answer B. {A, B, C}
Explanation
Start with {A}. A -> B adds B, and B -> C adds C. CD -> E cannot fire because D is not in the set, and nothing derives D. So {A}+ = {A, B, C}. Adding E is the common slip of applying CD -> E using C alone. Because the closure does not contain every attribute, A by itself is not a key of R.
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