Which set of ordered pairs represents y as a function of x? A. {(2, 5), (3, 1), (4, 1), (4, 7)}

B. {(3, 2), (4, 3), (3, 2), (2, 6)}

C. {(1, 3), (3, 5), (2, 5), (1, 6)}

D. {(6, 7), (4, 6), (-4, 4), (5, 1)}