18.56 If a square piece of paper is folded along the vertical midline, and then the folded paper is cut along the dotted line, three rectangular pieces of paper are obtained - one large and two small. What is the ratio of the perimeter of each small rectangle to the perimeter of the large rectangle?
(A) $\frac{1}{2}$.
(B) $\frac{2}{3}$.
(C) $\frac{3}{4}$.
(D) $\frac{4}{5}$.
(E) $\frac{5}{6}$.
(5th American Junior High School Mathematics Examination, 1989)
[Solution] The length of the large rectangle is 1, and the width is $\frac{1}{2}$ (assuming the side length of the original square is 1); the length of the small rectangle is 1, and the width is $\frac{1}{4}$. Therefore, their perimeter ratio is
$$
2 \cdot\left(1+\frac{1}{4}\right): 2 \cdot\left(1+\frac{1}{2}\right)=\frac{5}{6} .
$$

Thus, the answer is $(E)$.