Which of the following closed sets of ordered pairs is not a function?

A
{(3, 2), (4, 6), (5, 8)}

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

C
{(1, 2), (2, 5), (1, 3)}

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