18.38 As shown in the figure, $M$ is a point on side $CD$ of square $ABCD$, and also a point on side $AE$ of $\triangle ABE$, with $EM=3AM$. Then the ratio $S_{\square ABCD}: S_{\triangle ABE}$ is 
(A) $3: 2$.
(B) $2: 3$.
(C) $2: 1$.
(D) $1: 2$.
[Solution] Connect $B M$. By the property of the area of polygons, we have
$$
S_{\triangle A B C D}=2 S_{\triangle A B M}=2 \cdot \frac{1}{4} S_{\triangle A B E} \text {, }
$$

Therefore, $S_{\square A B C D}: S_{\triangle A B E}=1: 2$. Hence, the answer is $(D)$.