Read and fill in the gap with a suitable word

III. Read and fill in the gap with a suitable word.

            People need to breathe. If they don’t breathe, they (1)……………….die. But how to clean the air people breathe? If they breathe dirty(2) …………….., they will have (3)…………………….problems and become ill.Plants (4) ………………….. animals need clean air, too. A lot of things in our lives create harmful gases and (5)………………….the air dirty like cars, motorbikes and factories. Dirty air is called polluted air.