The binary stars A and B are seen orbiting each other with a period of 30 years. If the semi-major axis of their orbit is 10 AU, what is the combined mass of the two stars?

1. 0.5 solar mass
   
2. 1.1 solar mass
   
3. 1.9 solar mass
   
4. 10 solar mass
   
5. 15 solar mass