$17 \cdot 191$ In $\triangle A B C$, there is an interior angle of $36^{\circ}$, and a line $A D$ through vertex $A$ divides this triangle into two isosceles triangles. Then the number of different shapes of $\triangle A B C$ that satisfy the above conditions (considering similar triangles as the same shape) is
(A) 2 .
(B) 3 .
(C) 4 .
(D) 5 .
(China Guangzhou, Wuhan, Fuzhou, etc. five cities junior high school mathematics league, 1991)
［Solution］It is only necessary to note the following five cases: they are all $\triangle A B C$ that meet the requirements of the problem. Therefore, the answer is (D).