B the answer is b in this equation
A. Because the third laws say that for every action force the is an equal and opposite reaction force
When water evaporates of your body it cools you down, hence why after a shower when the water droplets begin to evaporate you feel cold
Well, if your question is how light affects plants,
then you would want to design an experiment that plays aruond with the amount of light a plant gets
thus the thing changing (or variable) would be amount of light