Which of these is a function?
{(0, 4), (0, 5), (0, 9), (0, 23)}
{(0, 1), (1, 2), (1, 5), (2, 9)}
{(0, 4), (2, 4), (3, 7), (4, 9)}
{(6, 0), (6, 1), (6, 7), (6, 11)}