There are n sets having 4 elements each. The difference set of any two of the sets is equal to one of the n sets. n can be at most ? (A difference set of A and B is (A∖B)∪(B∖A))<spanclass=′latex−bold′>(A)</span>3<spanclass=′latex−bold′>(B)</span>5<spanclass=′latex−bold′>(C)</span>7<spanclass=′latex−bold′>(D)</span>15<spanclass=′latex−bold′>(E)</span>None