In the diagram, ABCD is a rectangle. If the area of the circle is equal to the area of the shaded region, the radius of the circle is
Answer Choices
A. \sqrt{\frac6\pi}
B. \frac6\pi
C. \frac{6}{\sqrt{\pi}}
D. \sqrt{\frac{18}{\pi}}
E. \frac{18}{\pi}