17・112 In $\triangle A B C$, the heights on the three sides $a, b, c$ are $h_{a}=6, h_{b}=4, h_{c}=3$ respectively. The ratio of the three sides $a: b: c$ is 
(A) $1: 2: 3$.
(B) $2: 3: 4$.
(C) $3: 4: 6$.
(D) uncertain.
(Tianjin, China Junior High School Mathematics Competition, 1983)
[Solution] From $S_{\triangle A B C}=\frac{1}{2} a h_{a}=\frac{1}{2} b h_{b}=\frac{1}{2} c h_{c}$,
$$
a h_{a}=b h_{b}=c h_{c} .
$$

Also, $h_{a}: h_{b}: h_{c}=6: 4: 3$, then $a: b: c=2: 3: 4$. Therefore, the answer is $(B)$.