In the diagram, P Q is a diameter of a larger circle, point R is on P Q, and smaller semi-circles with diameters P R and Q R are drawn. If P R=6 and Q R=4, what is the ratio of the area of the shaded region to the area of the unshaded region?
Answer Choices
A. 4: 9
B. 2: 3
C. 3: 5
D. 2: 5
E. 1: 2