Question

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