Which set of ordered pairs is a function?
{(2, 1), (2, 2), (2,3), (2, 4), (2,5)}
{(0,0), (1,1), (1, -1), (4,2), (4,-2)}
{(1, 1), (2, 2), (3, 3), (4,4), (5,5)}
{(1,5), (2, 4), (3, 3), (2, 2), (1,1)}