4. Which of the following relations is NOT a function?
O {(2, 1), (4, 3), (6, 5), (8, 7)}
O {(2, 1), (4,3), (6, 5), (8,5)}
O {(2, 1), (4, 3), (6,5), (2, 7)}
O {(2, 1), (4, 1), (6, 5), (8, 7)}