Which relation is a function?

A. {(0, 1), (0, 2), (1, 2), (1, 3)}
B. {(8, 1), (4, 1), (0, 1), (–15, 1)}
C. {(–2, –12), (–2, 0), (–2, 4), (–2, 11)}
D. {(–4, –6), (–3, –2), (1, –2), (1, 0)}