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The earth is about 4,600 million years old. We humans have lived on the earth for only 35,000 years, but during this period we have changed our planet a lot in many ways.    1  
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, cars and buses have polluted the air. Factories have also polluted the land and the water.
  2  It is important to the earth. But now air pollution is destroying it and that has made a very big hole in the ozone layer (臭氧层).   3 This is very dangerous because it can cause cancer.
Carbon dioxide (CO₂) in the air has increased a lot.   4 it has formed a "blanket" (毛毯) around the each.    5  This makes our earth become warmer and warmer. Therefore, this causes the level of the ocean to rise.
All these problems are very serious. We must do something to stop our earth from being polluted anymore.

A. The heat from the sun can' t escape, so the temperature is rising. B. Too much harmful light from the sun passes through it and reaches the earth directly, C. Some things we've done are very good for the earth while some are bad. D. Around the earth, there’s a special kind of oxygen (O2) called "ozone" (O3). E. It mainly comes from different kinds of burning materials.