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Read the text carefully and then decide on the best word to write in each gap. Only put one word in each gap.

The Atmosphere of (1.) .

Would you like to (2.) an astronaut? To do this, you must travel at least 50 miles (80 kilometres) above the surface of the earth. But even after you have done that you are still (3.) earth's atmosphere. Indeed, there are at least 300 miles of air still between you and the true vacuum of space.

Most of what we think of as our atmosphere is actually the troposphere, that part of the atmosphere closest to earth. This is where most of our weather happens, and it is the only part of the atmosphere which has enough oxygen and warmth for humans to survive. (4.) part of the atmosphere is about ten miles thick at the equator and slightly half that height at the (5.) .

(6.) the troposphere is the stratosphere, which you have probably (7.) if you have travelled on an international jet liner. But there is another kind of 'jet' at this altitude. Huge rivers of air called 'jet streams' (8.) through the stratosphere, and the stratosphere contains the ozone layer which filters harmful ultraviolet rays which could (9.) make life on earth extinct. Above the stratosphere is the mesosphere, and above that the ionosphere, which is important for radio communications as signals can be (10.) off the ionosphere to different (11.) of the world.

Many people think that the atmosphere is mostly oxygen, (12.) that is what we breathe. But in (13.) oxygen makes up only about 21% of the atmosphere, and carbon dioxide, which we breathe out, makes up less than 1%. Over three quarters of the atmosphere is nitrogen, which was expelled from inside the planet while it was still very volcanically (14.) . We have a lot of nitrogen in our bodies, but we do not get it (15.) from the atmosphere. Instead we get our nitrogen from plants which we eat.


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