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

A.
{(3, 2) (4, 2) (3, 7)}

B.
{(8, 6) (6, 1) (‒1, ‒1)}

C.
{(2, ‒7) (3, ‒7) (4, ‒7)}

D.
{(‒5, 8) (3, -8) (‒8, 3)}