Which relation is also a function?
A. {(0, 3), (1, 2), (1, 4), (2, 1)}
B. {(-1, 3), (0, 2), (3, -1), (3, 2)}
C. {(2, -4), (2, -2), (2, 1), (2, 3)}
D. {(-4, -3), (0, -3), (1, -3), (4, -3)}