Which relation defined by a set of ordered pairs is a function? a {(2, 6), (3, 6), (4, 5), (6, 6), (6, 7)} b {(6, 2), (6, 3), (5, 4), (6, 6), (7, 6)} c {(2, 6), (3, 6), (4, 5), (6, 6), (7, 6)} d {(–2, 6), (3, –6), (–4, 5), (6, –6), (6, –7)}