9. A, C.

Let the average growth rate be $p$, then $(1+p)^{3}=\left(1+p_{1}\right)\left(1+p_{2}\right)\left(1+p_{3}\right) \leqslant$ $\left(\frac{3+p_{1}+p_{2}+p_{3}}{3}\right)^{3}$, hence $p \leqslant \frac{1}{3}\left(p_{1}+p_{2}+p_{3}\right)=\frac{1}{3}$.
9. The market annual demand for a product over the next four years forms a sequence $\left\{a_{n}\right\}(n=1,2,3,4)$, and it is predicted that the annual demand will increase by a percentage $p_{1}$ from the first year to the second year, by a percentage $p_{2}$ from the second year to the third year, and by a percentage $p_{3}$ from the third year to the fourth year, with $p_{1}+$ $p_{2}+p_{3}=1$. The following values are given: A. $\frac{2}{7}$; B. $\frac{2}{5}$; C. $\frac{1}{3}$; D. $\frac{1}{2}$; E. $\frac{2}{3}$. Which of these could be the annual average growth rate of the market demand over these four years? $\qquad$