Which set of ordered pairs below is a function?

1.) {(3 , 3), (6 , 1), (2 , 3)}
2.) {(4 , 7), (2 , 8), (4 , 2)}
3.) {(9 , 6), (3 , 1), (7 , 3)}

Please help!! I have been on this problem for awhile now and still can't figure it out.