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

{(1, 4), (1, 3), (1, 2), (1, 1)}

{(1, 3), (2, 6), (3, 9), (4, 12)}

{(3, 4), (2, 4), (5, 1), (3, 5)}