Which set of ordered pairs represents a function? A {(1, 8), (8, 1), (1, 5), (5, 1)} B {(2, 1), (3, 1), (4, 3), (5, 3)} C {(5, 1), (3, 8), (3, 7), (7, 1)} D {(5, 1), (5, 6), (1, 3), (9, 7)}