Which of these sets of ordered pairs below does not represent a function?
{(1, 1) (2, 2) (3, 3) (4, 4) (5, 5)}
{(2, 1) (4, 2) (6, 3) (8, 1) (10, 2)}
{(0, 0) (2, 3) (3, 6) (2, 9) (6, 12)}
{(1, 5) (2, 4) (3, 3) (4, 2) (5, 1)}