Which relation is a function?
{(3, 2), (4, 5), (6, 8), (7,5)}
B. {(3, 2), (4, 5), (7,-3), (7, -2)}
C. {(1, 4), (2, 5), (4, 7), (1, 9)}
D. {(2, 3), (5, 4), (2, -3), (0, -1)}