People need to breathe. If they don't breathe, they will die

Complete the following passage.

People need to breathe. If they don't breathe, they will die. But how clean is the air people breathe? If they breathe (1) __________ air, they will have breathing problems and become ill. Plants and animals need clean air too. A lot of things (2) __________ our lives create harmful gasses and make the air dirty, like cars, motorbikes and factories. Dirty air is called " (3) __________ air". Air pollution can also make our Earth (4) __________. The problem of air pollution started with the burning of coal in homes and factories.

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