Which Statement About Oxygen Levels on Earth Is True

question 68

Multiple Choice

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.

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