Which relation is a function?
(8,-4), (8,2), (3, -3), (3, 1), (-1,-1)
(2.0), (4,0), (2, 1), (4, 1), (2, 3)
(2, 1), (4,2), (6,4), (7,5), (8, 6)
(3, 8), (7, 6), (5,5), (4,4), (3, 2)