7・68 Given $y=a x^{5}+b x^{3}+c x-5$, when $x=-3$, $y=7$. What is the value of $y$ when $x=3$?  
(A) -3.  
(B) -7.  
(C) -17.  
(D) 7.  
(China Guangzhou, Wuhan, Fuzhou and other five cities junior high school mathematics league, 1984; China Anhui Province junior high school mathematics competition, 1997)
[Solution] Let $f(x)=a x^{5}+b x^{3}+c x$, then
$$
f(-x)=-f(x) .
$$

Given that when $x=-3$, $y=7$, i.e., $f(-3)-5=7$, we know $f(-3)=12$, and
$$
f(3)=-f(-3)=-12 \text {. }
$$

Therefore, when $x=3$, $y=f(3)-5=-17$.
Hence, the answer is $(C)$.