S \equal{} \{1,2,\dots,n\} is divided into two subsets. How the set is divided, if there exist two elements whose sum is a perfect square, then n is at least ?<spanclass=′latex−bold′>(A)</span>13<spanclass=′latex−bold′>(B)</span>14<spanclass=′latex−bold′>(C)</span>15<spanclass=′latex−bold′>(D)</span>16<spanclass=′latex−bold′>(E)</span>17