Which of the following sets of ordered pairs represents a function? A. {(-9,9), (-9,12), (-9,0), (12,-9)} B. {(0,-9), (14,-9), (0,9), (-9,14)} C. {(9,-9), (12,-9), (0,-9), (-9,12)} D. {(9,14), (9,4), (0,0), (11,16)}