Multiple Choice
By approximately ________ years ago, ________ had produced oxygen in enough amounts for it to make up a significant fraction of Earth's atmosphere.
A) 100 million; trees and plants
B) 1 billion; trees and plants
C) 250 million; bacteria and algae
D) 2.5 billion; bacteria and algae
E) 2,000; animals and humans
Correct Answer:
Verified
Correct Answer:
Verified
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