the production of nitrogen-fixing bacteria the mechanical weathering of roads the formation of acid rain the increase in runoff
I gave my students this multiple choice question: Scientists at a local university have been studying the impact that people have on Earth. One of the areas being studied is how the burning of fossil fuels affects the environment. Which effect of fossil fuel burning have the scientists most likely evaluated?

Only one answer is correct among these 4 choices:
- the production of nitrogen-fixing bacteria
- the mechanical weathering of roads
- the formation of acid rain
- the increase in runoff

Could you tell me which one is correct?

the formation of acid rain