The is a natural process in which certain gases in the atmosphere trap heat near earth and prevent it from radiating back into soace

The Greenhouse Effect is the natural process which traps heat on the surface of the Earth and prevents it from being radiated back to space. The "certain gasses" involved in this process are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N20), and fluorinated gasses. 

C.about 10% i think

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.