Answer the following question given this paragraph: 

Recall that air pollution is due to chemical substances and particles released into the air mainly by human actions. When most people think of air pollution, they think of the pollution outdoors. But it is just as easy to have indoor air pollution. Your home or school classroom probably doesn't get much fresh air. Sealing up your home reduces heating and cooling costs. But this also causes air pollution to stay trapped indoors. And people today usually spend a majority of their time indoors. So exposure to indoor air pollution can become a significant health risk.


Q: What is caused by human actions that releases chemical substances and particles into the air?

 Choices:

- CFO pollution

- Improper Emissions

- heavy pollution

- air pollution

A:
air pollution
