16.21 In $\triangle A B C$, $\angle A>\angle B>\angle C, \angle A \neq 90^{\circ}$, draw a line to divide $\triangle A B C$ into two parts, such that one part is similar to $\triangle A B C$. The number of such non-parallel lines is
(A) 3.
(B) 4.
(C) 5.
(D) 6.
(China Guangzhou, Wuhan, Fuzhou, and other five cities junior high school mathematics league, 1992)
［Solution］Such straight lines can be divided into three categories:
(1) Those parallel to one side of the triangle, totaling three;
(2) Two lines starting from the largest angle $\angle A$ meet the requirement, see the following figure (1) (note $\angle A$ $\left.\neq 90^{\circ}\right):$
(3) One line starting from the second largest angle $\angle B$ meets the requirement, see the above figure (2).
In summary, there are 6 such lines.
Therefore, the answer is $(D)$.