What is the least possible value of ab+bc+ac such that a2+b2+c2=1 where a,b,c are real numbers?<spanclass=′latex−bold′>a)</span>−1<spanclass=′latex−bold′>b)</span>−21<spanclass=′latex−bold′>c)</span>−31<spanclass=′latex−bold′>d)</span>−221<spanclass=′latex−bold′>e)</span>0