Which set of ordered pairs does not represent a function?
A {(9,4), (-4,6), (6,7), (8,7)}
B {(-7,1), (-7, -3), (1,7), (4,-8)}
C {(5,-6), (-1,-9), (-2, 1), (7,-3)}
D {(-4,-3), (4,7), (2,8), (-2,7)}