Which of the following relations is a function?

{(-4, 3), (-4, 2), (1, 9), (6, 7)}

{(-4, 3), (-2, 0), (2, 7), (3, 9)}

{(2, 1), (4, 2), (2, 3), (8, 4)}

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