Question

How did oxygen collect in the atmosphere?

Answer 1

Due to photosynthesis by green plants

Answer 2

Hey There
Heres ur answer.....
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Oxygen a very important thing for us Humans to Survive at...Without it we can even die... Now Question may arise that if we all use it then has it infinite supply?
well the answer is no. only 21%of atmosphere contains Oxygen
Now how does it return back?
Well we also have ans for that
It is being returned by plants in the process of Photosynthesis 
in which the extra oxygen is simply thrown out in atmosphere
But it constitutes around only 10 % of that 21% of oxygen
rest 90% of that 21% is being returned to atmosphere by algae
i know that sounds quite strange but thats truth.....
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Hope this helps.....!!!!

Thanks^_^