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.
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a. solvent b. solute c. antioxidant d. free radical e. acid
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Fill in the blank(s) with the appropriate word(s).