Answer:
Oxygen does not burn. It is not flammable, but it is an oxidizer.
Step-by-step explanation
I think you are misunderstanding what is meant by "fire burns up air". Fire does use oxygen when it burns. But it also produces carbon dioxide. An oxygen molecule has two oxygen atoms in it. Fires use this to produce carbon dioxide by adding a single carbon atom from the fire’s fuel (wood, for instance).
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