The early atmosphere of Earth had much less oxygen than today. How has Earth's oxygen abundance increased over Earth's history?
A. Ultraviolet light breaks up ozone into oxygen.
B. Volcanic eruptions release large amounts of oxygen.
C. Cometary impacts over Earth's history have contributed oxygen to the atmosphere.
D. Plant life generated oxygen through photosynthesis.
E. None of these choices are correct.
Answer: D
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