Which of the following relations is a function? {a. (1, 2), (1, 3), (2, 8), (3, 9) } b.{ (1, 2), (2, 3), (3, 4), (4, 5) } c.{ (3, -1), (2, 3), (3, 4), (1, 7) } d.{ (3, 0), (4, -3), (6, 7), (4, 4) }