Which of the following sets of ordered pairs is a function?

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

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

{(8, 1), (7, 0), (7, -1), (8, 2)}

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