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