A real-valued function f has the property that for all real numbers a and b,f(a+b)+f(a−b)=2f(a)f(b). Which one of the following cannot be the value of f(1)?<spanclass=′latex−bold′>(A)</span>0<spanclass=′latex−bold′>(B)</span>1<spanclass=′latex−bold′>(C)</span>−1<spanclass=′latex−bold′>(D)</span>2<spanclass=′latex−bold′>(E)</span>−2