The Earth is polluted（被污染）much. In some cities in the world, the air is dirty. People have to wmasks on their way to school or work.

But how does this happen? The awas not so dirty before. Cars and motorbikes are one of the reasons（原因）. Nowadays, there are more and more cars and motorbikes in the s. The sfrom them makes the air very dirty. So we should susing more cars. We should take buses or the underground more often to school or work. If we go to a place nearby, walking is a good way. It can also make us hand strong. So let's do something to make the air clean.

We can get wood from trees.

So please plant more trees.

Trees make our city beautiful.

They keep the air clean and cool.

We can use wood to make paper, desks and chairs.

The Earth is a place. There are , rivers, mountains and . Some parts of the Earth are hot, but some parts are very cold. There are many kinds of and plants on the Earth. Some are , but some are small. Today some parts of the Earth are very . People throw into the rivers. Some people cut down too many trees. They pollute the , the water and the . It is important for us to the Earth.