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