Two circles of radius 5 are externally tangent to each other and are internally tangent to a circle of radius 13 at points A and B, as shown in the diagram. The distance A B can be written in the form \dfrac{m}{n}, where m and n are relatively prime positive integers. What is m+n ?
Answer Choices
A. 21
B. 29
C. 58
D. 69
E. 93