After a decline to an historic low about 300 million years ago, carbon dioxide levels in the Earth's atmosphere rose again for a while. Hypothesize a reason why this occurred.
A. Higher oxygen levels resulted in the proliferation of aerobic bacteria; their respiration released large amounts of carbon dioxide.
B. Higher oxygen levels promoted large-scale fires across the landscape; the breakdown of organic molecules in these fires released carbon dioxide.
C. Higher temperatures caused an increase in plant growth, resulting in an increase in the amount of carbon dioxide synthesized.
D. Lower temperatures resulted in reduced plant growth; with less vegetation to absorb carbon dioxide, the carbon dioxide levels rose.
E. All the vegetation that had been produced prior to the historic low decayed, releasing carbon dioxide.
Answer: D
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