In the diagram, the two circles are centerd at O. Point S is on the larger circle. Point Q is the point of intersection of OS and the smaller circle. Line segment PR is a chord of the larger circle and touches (that is, is tangent to) the smaller circle at Q. Note that OS is the perpendicular bisector of PR. If PR=12 and QS=4, then the radius of the larger circle is
Answer Choices
A. 6.0
B. 5.0
C. 6.5
D. 7.2
E. 20.0