Which statement about oxygen levels on Earth is true?
A) The atmosphere of Proterozoic Earth contained almost as much free oxygen as present-day Earth, but most of this oxygen was lost in the Cambrian.
B) Early photosynthetic bacteria generated free oxygen as a by-product of the chemical splitting of water.
C) The oxygen-generating cyanobacteria that existed on early Earth have gone extinct.
D) Most of early Earth's atmospheric oxygen arose via land plants.
E) Today's atmospheric oxygen levels are the highest they have ever been.
Correct Answer:
Verified
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