Which set of ordered pairs represents a function?
A. {(5, 1), (5,2), (5, 3), (5, 4), (5,5)}
B. {(0,7), (2, 7), (4,7), (6, 7), (8, 7)}
c. {(1, 2), (2, 3), (3, 2), (2, 1), (1, 0))
D. {(1,4), (2, 4), (3, 4), (4,3), (4, 2)}