If $f(x)$ in continuous in the clesed interval $l a, b 1$, differentiable in $(n, b)$ and $f(\alpha)=f(b)$, then there esikts at least one value c of $x$ in $(a, b)$ such that $f^{\prime}(c)$ is equal to
(A) 1
(B) $-1$
(C) 2 $\begin{array}{ll}\text { (D) } 0 . & \left(V, \tau, U_{.}, 2009\right)\end{array}$