18.100 Construct a trapezoid with the segments $a=5, b=10, c=15, d=20$. The number of such trapezoids (not congruent)
(A) can be at least three.
(B) can be exactly two.
(C) can be only one.
(D) cannot be made at all.
（1st "Five Sheep Cup" Junior High School Mathematics Competition, 1989）
［Solution］Since a trapezoid always has two sides parallel, thus:
If $a // c$, as shown in Fig. (1), $10,10,20$ cannot form the three sides of a triangle, so it is not suitable; if $a // b, b // c, c // d, b // d$, for the same reason as above, it is not suitable; while $a // d$ can form two trapezoids, but they are congruent, see Fig. (2) ～(3). Therefore, the answer is $(C)$.