Answer:
Humans
Step-by-step explanation:
The main cause of pollution is the burning of fuel, over-use of fertilizers and pesticides, carelessness, and the improper disposal of waste. And who does this? Not casper, not pennywise, not animals, but us humans. So we are the main reasons why pollution exist.
You can just write what causes pollution (the underlined), you dont need to write about who causes it.
Hope this helped!
Have a supercalifragilisticexpialidocious day!