To revolve means to a. keep completely still b. move around another object c. preserve or save something d. spin around in one place

B, move around another object.

The correct answer would be the second choice, B. move around another object.
To revolve means to move around another object. The other choices are not correct, except for D, which is somewhat true, but not completely. 

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.