I would say C is the most correct.
In D it depends on what water source you're using. Let's say it is a waterfall, then the source of the water (melting ice or a lake) may disappear in the future.
If you're using underwater "windmills" placed in the ocean, then you would expect it to last a while as the ocean will not disappear in the near future.
Answer:
Velocity is the distance traveled during a specific unit of time
The air in the sky and the clouds are gas, however some things in the sky [such as the sun/moon] are solid.
Hope that helped!
