Which relation is a function? {(1, 3), (2, 6), (3, 9), (4, 12)} {(3, 4), (3, 3), (1, 2), (1, 5)} {(1, 4), (1, 3), (1, 2), (1, 1)} {(3, 4), (2, 4), (5, 1), (3, 5)}