The presence of free oxygen in the atmosphere is attributed mainly to Volcanic activity Development of plant life, especially algae Formation of the inner core Formation of the outer core

Development of plant life, especially algae

Explanation:

The main source of oxygen in the atmosphere is the photosynthesis process that produces sugars and oxygen by utilizing carbon dioxide and water.Tiny oceanic plants called phytoplanktons  have the ability to support life in water bodies for plants, animals and fish.The phytoplanktons consume most of the carbon dioxide in air and with the help of energy from the sun, they convert nutrients and carbon dioxide to complex organic compounds which are main source of plant  material. Phytoplanktons are responsible for oxygen in water which is approximated to be 50% of world's oxygen.Green-algae is a good example of phytoplankons.

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