17・194 As shown in the figure, $E$ is the intersection of the diagonals of the convex quadrilateral $A B C D$, $F$ is on the side $B C$, $A B / / C D / / E F$, the area of $\triangle B C E$ is $S$. Then the number of triangles in the figure with an area of $S$ is
(A) 1.
(B) 2.
(C) 3.
(D) 4.
(3rd "Five Sheep Cup" Junior High School Mathematics Competition, 1991)
［Solution］From the area formula of a triangle and the given conditions, we have
$$
S_{\triangle A F C}=S_{\triangle B C E}=S \text {. }
$$

Similarly,
$$
S_{\triangle D F B}=S \text {. }
$$

Also, $S_{\triangle A B C}=S_{\triangle A B D}$, so $S_{\triangle A D E}=S$. Thus, there are 4 triangles in the figure with an area of $S$. Therefore, the answer is $(D)$.