What is the leading cause of water contamination in countries without water treatment plants?

Factories which leaves out smokes (carbon dioxide and carbon monoxide), carbon dioxide mix up with sulfur and acid rain is created which damages out skin permanently and carbon monoxide which very harmful and since they are minute particle they can pass inside our body through nose and takes out oxygen from our body, the only thing left in our body will be carbon which leads to death.

The answer would be c