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Until the early _______ there were only negligible amounts of free oxygen in the atmosphere.

A. Cenozoic
B. Mesozoic
C. Proterozoic
D. Precambian

User Himayan
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2 Answers

1 vote
C Proterozoic is the answer I just took the test hope this is very help 
User Brian Gesiak
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Just took the test it's D. proterozoic
User Andrii Shvydkyi
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