Which relation is a function?

help me

A.

{(7, 12), (1, –5), (3, –10), (2, –5)}


B.

{(1, 2), (1, 5), (1, –1), (1, 4)}


C.

{(–3, 4), (–2, 5), (0, 9), (0, 12)}


D.

{(8, –1), (2, –1), (3, 8), (2, 5}