Which of the following relations is a function?

{(-2, 3), (4, 1), (0, 0), (-2, 2)}
{(1, 1), (2, 2), (1, 2), (2, 1)}
{(2,3), (3, 2), (6, 3), (3, 6)}
{(6,5), (5, 6), (3, 1), (1,3)}