Question

Which set of ordered pairs would be considered a function?
{(2,3), (3, 4), (2, 5), (5,3)}
{(1,0), (2,4), (3, 6), (4,9))
(-1,2). (-1,3), (0,3), (4.6))
{(6,10). (-2,5), (7,9), (6,12)}