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) }