Which of the following sets does NOT represent a function.
O {(1,9), (-1,-9),(2,9), (-2, -9) }
O {(1,-9), (2,-5), (-1,9), (-2,5)}
O {(1,9) (-1,5), (2, -9), (-2,-5) }
O {(1,9), (1,-9), (2,5),(2,-5) }