Agricultural techniques like irrigation could a civilization expand its borders because they could use irrigation a. lessen the threat of flooding by spreading the water thin. b. protect their cities by building defensive dams and dikes. c. develop more land that was previously too dry for farming. d. send convoys to conquer areas previously unreachable by land.

I think the correct answer among the choices listed above is option C. Agricultural techniques like irrigation could help a civilization expand its borders because they could use irrigation to develop more land that was previously too dry for farming. 

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.