18.44 In rhombus $ABCD$, a line segment parallel to diagonal $BD$ is drawn. When the length of this line segment is viewed as a function of the distance from point $A$ to this line segment, its graph is
(A) a straight line passing through the origin.
(B) a straight line intersecting the positive directions of both coordinate axes.
(C) a "V" shaped broken line segment.
(D) a "$\wedge$" shaped broken line segment.
(E) none of the above.
(5th American High School Mathematics Examination, 1954)
[Solution] Consider two cases:
For figure (1), we have
$$
\frac{l}{t}=\frac{2 B O}{A O}, 0<t \leqslant|A O|,
$$

which gives $\quad l=\frac{2 B O}{A O} t, 0<t \leqslant|A O|$.
For figure (2), similarly, we have
$$
\begin{array}{l}
l=\frac{2 B O}{A O}(A C-t), \\
|A O| \leqslant t<1 A C \mid .
\end{array}
$$

In summary, the graph of the function is shown in figure (3). Therefore, the answer is $(D)$.