21. A rectangle is placed obliquely on top of an identical rectangle, as shown.
The area $X$ of the overlapping region (shaded more darkly) is one eighth of the total shaded area.
What fraction of the area of one rectangle is $X$ ?
A $\frac{1}{3}$
B $\frac{2}{7}$
C $\frac{1}{4}$
D $\frac{2}{9}$
E $\frac{1}{5}$