Which of the following relations describes a function? a. { (0, 0), (1, 1), (4, 2), (9, 3) } b. { (0, 0), (1, 1), (1, -1), (4, 2) } c. { (0, 0), (1, 1), (2, -2), (2, 2) } d. { (-4, 2), (-1, -1), (-1, 1), (0, 0) }