Find the Average Rate of Change of F(x)=3 \Cos $\left[\frac{\pi}{3}, \frac{2 \pi}{3}\right]$

Multiple Choice

Find the average rate of change of f(x) =3 \cos x over the interval $\left[\frac{\pi}{3}, \frac{2 \pi}{3}\right]$

A) -3
B) $-9 \pi$
C) $-\frac{9}{\pi}$
D) $\frac{\pi}{3}$