If one can find a student with at least k friends in any class which has 21 students such that at least two of any three of these students are friends, what is the largest possible value of k?<spanclass=′latex−bold′>(A)</span>8<spanclass=′latex−bold′>(B)</span>9<spanclass=′latex−bold′>(C)</span>10<spanclass=′latex−bold′>(D)</span>11<spanclass=′latex−bold′>(E)</span>12