Question

The first major source of oxygen gas was _____.

a. volcanic outgassing
b. meteorite impacts
c. cyanobacteria
d. banded iron formations

Answer 1

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".

Answer 2

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.