Which set is a function?
A) {(0,3), (3,0), (0,4), (4,0)}
B) {(0,2), (2,0), (4,6), (6,4)}
C) {(2,6), (3,6), (4,6), (2,0)}
D) {(6,2), (2,0), (4,6), (6,4)}