Two distinct lines pass through the center of three concentric circles of radii 3, 2 , and 1 . The area of the shaded region in the diagram is 8 / 13 of the area of the unshaded region. What is the radian measure of the acute angle formed by the two lines? (Note: radians is 180 degrees.)
Answer Choices
A. \frac{\pi}{8}
B. \frac{\pi}{7}
C. \frac{\pi}{6}
D. \frac{\pi}{5}
E. \frac{\pi}{4}