The Concentration of Oxygen in the Earth's Atmosphere

question 122

Multiple Choice

The concentration of oxygen in the Earth's atmosphere

• A) has increased steadily through time.
• B) has decreased steadily through time.
• C) has been both higher and lower in the past than at present.
• D) was lower during most of the Permian than at present.
• E) was at its highest levels in the Cambrian.

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