Two circles C_1 and C_2 touch each other externally and the line l is a common tangent. The line m is parallel to l and touches the two circles C_1 and C_3. The three circles are mutually tangent. If the radius of C_2 is 9 and the radius of C_3 is 4, what is the radius of C_1?
Answer Choices
A. 10.4
B. 11
C. 8\sqrt2
D. 12
E. 7\sqrt3