Which relation is NOT a function? Explain your answer.
(1) {(1,2), (3,4),(4,5), (5,6)}
(2) {(3,1),(2,1),(1,2), (3,2)}
(3) {(4,1),(5,1),(6,1),(7.1)}
(4) {(0,0), (1,1),(2,2), (3,3)}