Which of the following sets of ordered pairs is a function?
F {(1, 1), (2, 1), (3, 2), (4,4)}
G {(1,2), (2, 2), (3, 3), (2,4)}
H {(2, 1), (2, 2), (3, 4), (5, 6)}
J {(-2,-1), (1, 2), (3, 4), (1,5)}​