Which of the following are true?
i. the greenhouse effect and global warming are the same thing.
ii. without the greenhouse effect there would be no life on earth.
iii. some of the radiation flowing from the earth to space is absorbed by greenhouse gasses and re-radiated back to the surface and out to space.
iv. co2 makes up about 96% of mars atmosphere

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.

Cause of he different elevations  hope this ?