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