Which relation is a function?

A. {(-1, -2), (1, 3), (1, 5), (3, 5)}
B. {(0, 2), (1, 2), (0, -5), (4, -1)}
C. {(0, 5), (1, 5), (4, 8), (6, 1)}
D. {(0, 4), (1, 2), 0, -5), (2, 1)}