Question

Which process do scientists think provided Earth with an oxygen-rich atmosphere?

Answer 1

photosynthesis

So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff? The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen.

Step-by-step explanation:

Answer 2

photosynthesis

Step-by-step explanation:

Photosynthesis: Is the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct.