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The earth is about 4，600 million years old.And we have changed our planet a lot in many ways. 

All over the world，people have cut down millions of trees，so more and more kinds of animals and plants are disappearing.In big cities，factories have polluted the land and the water. Around the earth，there's a special kind of oxygen (O₂) called "ozone (O₃)"． But now air pollution is destroying it and has made a very big hole in the ozone layer (臭氧层).Too much harmful light from the sun passes through it and reaches the earth directly.This is very dangerous because it can cause cancer.

Carbon dioxide (CO₂) in the air has increased a lot.It mainly comes from different kinds of burning materials.It has formed a "blanket (毛毯)" around the earth.The heat from the sun can't escape，so the temperature is rising. Therefore，this causes the level of the ocean to rise.

 We must do something to stop our earth from being polluted.

A．All these problems are very serious. B．We need to recycle some rubbish. C．Some things we've done are very good for the earth while some are bad. D．It is important to the earth. E．This makes our earth become warmer and warmer. F．Cars and buses have polluted the air. G．A lot of people die of air and water pollution.