The Great Lakes and Corn Belt occupies a significant portion of this subregion located between the Appalachian Mountains to the east and the Rocky Mountains to the west. A. America’ s central lowlands
B. America’ s driftless area
C. Canada
D. America’ s riverine highlands

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.