Big cities are always densely populated. You will never find a city without any plant or factory. They may produce necessary goods, but at the same time they are the main air pollutors in a city. This can become a global ecological problem if we don't invent the way of cleaning the air. Plants and factories also pollute water in rivers and lakes. A polluted river loses its "population": fish and water grass, and becomes unavailable for rest and fishing. While I am speaking about factories I don't except that people are the city polluters too. Nowadays every fourth person owns a car, which also pollutes the air. There are some people who drop litter in parks and even in streets. This litter will not disappear immediately. (время заканчивается, сейчас добавлю остальное)
Plants and factories also pollute water in rivers and lakes. A polluted river loses its "population": fish and water grass, and becomes unavailable for rest and fishing.
While I am speaking about factories I don't except that people are the city polluters too. Nowadays every fourth person owns a car, which also pollutes the air. There are some people who drop litter in parks and even in streets. This litter will not disappear immediately. (время заканчивается, сейчас добавлю остальное)