The Caucuses Mountains is a natural boundary for every country EXCEPT: Georgia
Georgia

Kazakhstan

This is all you, but i can give you some topics that you could try. if it rained when you want to do something outdoors, it 'limits' that human activity. maybe you want to do an experiment that goes throughout the whole year and you have to have above freezing the entire year. this 'limits' your activity. i thought of those just off of the top of my head. something that gets to me that might be getting to you is you are thinking too hard about what to do. think simply, and if it is too simple, then bump it up just a little bit.

The sun's parallax would be 180 degrees, greater than which you can't get.  so a. would be your anwser 

Themes; subfields is the answer