Which relation is a function?

A. {(3, 2), (2, 1), (-1, 2), (3, 3)}

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

C. {(5, 0), (0, 1), (5, 2), (4, 4)}

D. {(0, 1), (-4, 1), (4, 2), (-4, -1)}