The sets of ordered pairs below represent relations.

I {(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)}
II {(1, 2), (2, 1), (1, 3), (0, 1), (3, 1)}
III {(0, 2), (1, 2), (2, 4), (3, 4), (3, 6)}
IV {(1, 6), (2, 6), (3, 6), (4, 6), (5, 6)}
Which of these sets are also functions?

I only

I and IV

II and III

II, III, and IV