Let `f(x)` be a differentiable function and `f(alpha)=f(beta)=0(alpha< beta),` then the interval `(alpha,beta)`
A. f(x) + f(x) =0 has at least one root
B. f(x) -f(x) =0 has at least one real root
C. f(x) ,f(x) =0 has at least one real root
D. none of these
