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Kalahira

The word that best fits in the blank in the sentence, "The first major source of oxygen gas was _____" is cyanobacteria. Thus, the answer is letter C. Cyanobacteria are blue-green algae and they are believed to have caused the "great Oxidation Event".

W0lf93

The first major source of life on Earth was c. cyanobacteria. These organisms produce food by using energy from the sun; they are photosynthetic organisms. Over millions of years these organisms used CO2 in the air for photosynthesis and released oxygen as a byproduct. Over time oxygen concentrations rose high enough to support life.

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