Consider all quadrilaterals A B C D such that A B=14, B C=9, C D=7, and D A=12. What is the radius of the largest possible circle that fits inside or on the boundary of such a quadrilateral?
Answer Choices
A. \sqrt{15}
B. \sqrt{21}
C. 2 \sqrt{6}
D. 5
E. 2 \sqrt{7}