Which of the following cannot be equal to x2+y2, if x2+xy+y2=1 where x,y are real numbers?<spanclass=′latex−bold′>a)</span>21<spanclass=′latex−bold′>b)</span>21<spanclass=′latex−bold′>c)</span>2<spanclass=′latex−bold′>d)</span>3−3<spanclass=′latex−bold′>e)</span>None of above