Most of the Earth's oxygen occurs in the form of minerals in the crust and mantle. The oxygen that is available in the atmosphere is used primarily by plants, animals, and bacteria to perform cellular respiration.
Why is running out of oxygen still not a real concern?

A. Oxygen is recycled through the interrelated processes of photosynthesis and cellular respiration.
OB. Oxygen can easily be mined from rocks and sand and then be released into the atmosphere.
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OC. The Sun creates more than enough new oxygen atoms in the atmosphere by splitting phosphate molecules.
OD. Lightning creates large quantities of ozone, which quickly breaks down into oxygen molecules.
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