If "less activites" caused the change in air pollution, What are some of these "activites" that you speak of? Name THREE.

Using fossil fuels in factories, less people out using cars (so less exaust) and also less trash in waters so the evaporated water isn't so full of bacteria.

Explanation:

IDK if this is what you are looking for but, less bacteria and smoke and fossil fuels go into the air making the air cleaner.

I'm open to questions.

answer; (d) melting of sea ice;