$17 \cdot 1$ If two triangles have two sides and the altitude on one of the sides respectively equal, then the relationship between the angles opposite to the third sides of these two triangles is
(A) equal.
(B) complementary.
(C) supplementary.
(D) equal or supplementary.
(1st China Tianjin "Chinese Youth Cup" Mathematics Invitational Competition, 1987)
[Solution] From the figures (1) and (2) below, we can see that the angles opposite the third sides of the two triangles are equal; from figures (1) and (3), we can see that the angles opposite the third sides are supplementary. Therefore, the correct choice is $(D)$.