Which set of ordered pairs represents a function?

A. {(-7, 3), (-7, 6), (-7, 12), (-7, 18)}

B. {(9, -3), (4, -2), (1, -1), (0, 0), (1, -3)}

C. {(-1, 3), (2, 6), (3, 9), (4, 12), (5, 20)}