which function is NOT one to one?
1) {(0,1), (1,2), (2,3), (3,4)}
2) {(0,0),(1,1),(2,2),(3,3)}
3) {(0,1), (1,0), (2,3), (3,2)}
4) {(0,1), (1,0), (2,0), (3,2)}