Because in atmosphere there will not be sufficient oxygen which we require to survive.. Hence Hope u understood it....I have told it in the simplest way possible Pls mark as brainliest if u understood it correctly..
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there is no air and no oxygen.. not the question of sufficiency...

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Astronauts carry oxygen cylinder with them to space because as we know there is no air in the space and all the living beings need oxygen to live.
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