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