Which of the following set of ordered pairs represents Y as a function of X

The answers are
1 {(1,2),(1,3),(1,4),(1,5)}
2{(2.5,8),(3.5,8),(2.5,2),(4.2)}
3{(-1,1),(0,0),(1,1),(2,2)}
4{(4,-2),(1,-1),(0,0),(4,2)}