D because friction will slow it down going up hills (for safety reasons) and you go really fast going down.
It makes it easier to see in dark places
The answer for Plato users is C. It is the energy that is required to overcome mass defect.
From Plato "Nuclear binding energy is the energy that is required to overcome the energy that is used to keep the nucleus together, the mass defect."
The number of times something happens in one second is its "frequency".