which of the ordered pairs is not a function?

(1) {(0,9),(9,0),(1,2),(3,4)}

(2) {(0,1),(-1,0),(1,2),(3,2)}

(3) {(2,3),(3,4),(4,5),(5,6)}

(4) {(2,3),(2,4),(4,5),(4,6)}