Question

Three billion years ago plants began the process of photosynthesis. Why would
the appearance of plants that use photosynthesis to produce their food affect the
increase in atmospheric oxygen and the decrease in carbon dioxide?

Answer 1

because a chemical reaction began to happen at a very big rate which uses CO2 as a reactant and O2 as product

6CO2+6H20--->C6H12O6+6O2