Answer:
Human is the main problem.
Step-by-step explanation:
The author relate the ideas of pollution that greatly damaged our environment. He says that there is high pollution in the cities which causes more smog and more diseases by inhaling dirty air of the surrounding. Water bodies such as lakes, streams and livers is polluted and water is undrinkable due to this pollution caused by human activities so the author wants to say that human activities is the main reason for all the problems in the environment.