A scientist is analyzing air bubbles trapped in ancient rocks that can be used to understand the ancient atmosphere. What do you expect the scientist will observe when comparing rocks younger than 3.8 billion years old to rocks older than 3.8 billion years?
A. The ratio of carbon-12 will be lower in the older rocks, because carbon-12 had to build up on earth before life could evolve.
B. The amount of carbon will vary unpredictably in the younger rocks, because rock weathering added carbon dioxide to the atmosphere, and volcanic activity sequestered it.
C. The younger rocks will have higher levels of carbon dioxide, because there was no carbon dioxide in the atmosphere before animals evolved and expelled it as waste.
D. The ratio of the carbon-12 isotope will be lower in the younger rocks, because living organisms preferentially use carbon-12 over other isotopes.
Answer: D
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