Which of the following cannot be equal to x2+4x1 where x is a positive real number?<spanclass=′latex−bold′>a)</span>3−1<spanclass=′latex−bold′>b)</span>22−2<spanclass=′latex−bold′>c)</span>5−1<spanclass=′latex−bold′>d)</span>1<spanclass=′latex−bold′>e)</span>None of above