17-196 Given the lengths of five line segments are $3,5,7,9,11$. If each time three of these line segments are used as sides to form a triangle, then the maximum number of non-congruent triangles that can be formed is
(A) 10.
(B) 7.
(C) 3.
(D) 2.
(China Guangzhou, Wuhan, Fuzhou, etc. Five Cities Junior High School Mathematics League, 1992)
[Solution] According to the properties of the three sides of a triangle, the groups of line segments that can form the three sides of a triangle are
$$
\begin{array}{l}
3,5,7 ; \quad 3,7,9 ; \quad 3,9,11 ; \quad 5,7,9 \text {; } \\
5,7,11 ; \quad 5,9,11 ; \quad 7,9,11 . \\
\end{array}
$$

Therefore, the answer is $(B)$.