將段落中的劃線句子翻譯成漢語。As we all know，the air is not as fresh as it was before，especially in the big cities.Factories，cars and planes are making the air dirty. （1）In the morning you can see the dark smoke as clouds floating over the houses.Sometimes it makes it difficult for people to breathe. Now even those who are not scientists have begun to pay attention to air.We realize that air does not contain the same things that it contained years ago.Dust and waste gases from car engines，planes and factories have been adding to it.It is time to learn how to protect the air，the roof over the people on the earth.