Which relation is a function? a. {(1, 2); (1, 3); (1, 4); (1, 5)} b. {(1, 2); (2, 3); (3, 4); (1, 5)} c. {(1, 2); (2, 2); (3, 2); (4, 2)} d. {(1, 2); (2, 3); (3, 4); (2, 5)}

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