Consider the following relation R(M N P Q) with functional dependencies:

FD =  {MN → PQ, P → M, Q → N} 

Which of the following is the correct set of candidate keys?
(A) 2
(B) 4
(C) 3
(D) 5


Answer: (B)

Explanation: Given FDs:
MN → PQ, P → M, Q → N
P → M
It can be replaced in MN → PQ
PN → PQ
[PN]⁺ → MNPQ
Q → N
Replace it
[PQ]⁺ → MNPQ
[MQ]⁺ → MNPQ

hence, there are 4 candidate keys.

Option (B) is correct.

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