What component of Earth's atmosphere exists entirely as a result of photosynthesis?

A) oxygen gas

B) water vapor

C) nitrogen gas

D) carbon dioxide gas

Oxygen in the atmosphere is entirely the result of photosynthesis. Photosynthesis is the process by which plants use sunlight to make food. A byproduct of this reaction is oxygen. Oxygen in our atmosphere only appeared after the first photosynthetic organisms appeared on earth. Examples of these are stromatolites.

The Earth's atmosphere is composed of many different molecules including the gases and the vapors. There are many different types of gases that occur in the atmosphere including nitrogen, oxygen, argon, and other which are present in trace percentages only. The gas that occurs as a result of photosynthesis is the oxygen gas. The answer is letter A.