Scientists believe that the earth's early atmosphere lacked gas because photosynthetic organisms did not yet exist. a hydrogen b nitrogen c oxygen d sulfur

Photosynthesis is performed by plant and algae and results in the production of oxygen gas, which is utilized by other organisms in respiration. So, earth atmosphere lacked oxygen when there were no photosynthetic organisms and other living beings were thus not able to thrive. The plants and algae uses sunlight and carbon for producing glucose, and oxygen is released.