It is difficult to estimate when cyanobacteria appeared on Earth using evidence from the oxidation state of the Earth's crust, because
A) several abiotic reactions could have formed Fe3+.
B) there are multiple forms of anoxygenic photosynthesis.
C) there are multiple groups of oxygenic photosynthetic bacteria.
D) several ancient anaerobic metabolisms form Fe3+.
Answer: D
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