$$
\begin{array}{l}{\text { Multiple Choice If } f(x)=\cos x, \text { then the Mean Value }} \\ {\text { Theorem guarantees that somewhere between } 0 \text { and } \pi / 3, f^{\prime}(x)=} \\ {\text { (A) }-\frac{3}{2 \pi} \quad \text { (B) }-\frac{\sqrt{3}}{2} \quad(\mathbf{C})-\frac{1}{2} \quad \text { (D) } 0}\end{array}
$$