Which of the following sets of ordered pairs defines a relation that
is NOT a function?
O {(1, 5), (5, 1), (7, 9), (9, 7)}
O {(6, 7), (8, 10), (13, 9), (21, 10)}
O {(2, 2), (3, 4), (5, 8), (12, 16)}
O {(5, 14), (5, 16), (7, 23), (11, 31)}