The proportion of oxygen in our atmosphere today

A. has increased gradually over the past billion years as a result of cyanobacteria.
B. has increased to 20% since before photosynthesis.
C. has remained about the same since the first organisms developed.
D. has decreased to 10%, from higher levels before photosynthesis.
E. increased rapidly since the development of water-splitting photosynthesis and eukaryotes.


A. has increased gradually over the past billion years as a result of cyanobacteria.

Biology & Microbiology

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Biology & Microbiology