1. Which of the following sets of ordered pairs are functions?

a{(1, 1), (2, 4), (3, 3), (3, 6), (4, 8)}

b{(1, 3), (2, 5), (3, 7), (4, 9), (5, 11)}

c{(2, 2), (1, 1), (0, 0), (1, 1), (2, 2)}