Which set of ordered pairs represents y as a function of x?
a) {(2, 5), (3, 1), (2, 1), (4, 7)}
b) {(3, 2), (4, 3), (5, 2), (2, 6)}
c) {(1, 3), (3, 5), (2, 5), (1, 6)}
d) {(4, 7), (4, 6), (4, 4), (4, 1)}