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Which two processes are responsible for keeping the percentage of atmospheric oxygen at relatively constant levels?

Respuesta :

I believe the two processes which the question is talking about is photosynthesis and cellular respiration. These processes are responsible for keeping an organism alive and living. Photosynthesis makes so the plant can live off of sunlight, water, and  oxygen. Cellular respiration make so that nutrient and oxygen transport can occur. 
photosynthesis and respiration  are responsible for it
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