$13 \cdot 31$ The maximum value of the function $y=-x-\frac{9}{x}+18(x>0)$ is
(A) 24 .
(B) 18 .
(C) 12 .
(D) 2 .
(China Guangzhou, Wuhan, Fuzhou, etc. five cities junior high school mathematics league, 1991)
[Solution] Let $y=-\left(x+\frac{9}{x}\right)+18, x>0$. When $x>0$, $x+\frac{9}{x} \geqslant 2 \sqrt{x \cdot \frac{9}{x}}=6$. Therefore, $y \leqslant-6+18=12$. That is, $y_{\text {max }}=12 \quad(x>0$  when $)$. Hence, the answer is $(C)$.