18. A square has two of its vertices on a semicircle and the other two on the diameter of the semicircle as shown. The radius of the circle is 1 . What is the area of the square?
A $\frac{4}{5}$
B $\frac{\pi}{4}$
C 1
$\mathrm{D} \frac{4}{3}$
$\mathrm{E} \frac{2}{\sqrt{3}}$