I believe it is A because it makes the most sense ?
If you are talking about heart rate zones, it would be best to stay within the aerobic zone which is about 70%-80% of your maximum heart rate and you are able to sustain the exercise effort for a long period of time and gain more health benefits of improving your cardiovascular self.
Stop using fossil fuel and start using green energy from dams, windmills, solar panels, ect...
I believe its B. radioactive waste