17.123 Let $O$ be an interior point of $\triangle A B C$, $s_{1}=O A+O B+O C$, $s_{2}=A B+B C+C A$, then
(A) Regardless of what $\triangle A B C$ is, $s_{2}>2 s_{1}$ and $s_{1} \leqslant s_{2}$.
(B) Regardless of what $\triangle A B C$ is, $s_{2} \geqslant 2 s_{1}$ and $s_{1}\frac{1}{2} s_{2}$ and $s_{1}<s_{2}$.
(D) Regardless of what $\triangle A B C$ is, $s_{2} \geqslant 2 s_{1}$ and $s_{1} \leqslant s_{2}$.
(E) None of the above (A), (B), (C), (D) are always true for any triangle.
(17th American High School Mathematics Examination, 1966)
［Solution］From the relationship between the sides and angles of a triangle, we have
$$
\begin{array}{l}
A B < O A + O B < A C + C B, \\
B C < O B + O C < B A + A C, \\
C A < O C + O A < C B + B A .
\end{array}
$$

Adding the above inequalities respectively, we get $s_{2} < 2 s_{1} < 2 s_{2}$, which simplifies to $\frac{1}{2} s_{2} < s_{1} < s_{2}$.

Therefore, the answer is $(C)$.