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