The study of the location of fracture zones has provided evidence to support the theory of plate tectonics because a. fracture zones indicate the locations of once-active underwater faults. b. fracture zones prevent subduction at ocean trenches. c. fracture zones allow old, oceanic crust to be subducted by continental crust. d. fracture zones indicate that tectonic plates are always stationary.

 fracture zones allow old, oceanic crust to be subducted by continental crust. 

The study of the location of fracture zones has provided evidence to support the theory of plate tectonics because fracture zones are the zones where new oceanic crust is formed, allowing old oceanic crust to be subducted by continental crust, thus creating a circle in the crust that is a piece of evidence in support of the theory of plate tectonics. 

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.