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