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