Answer:
b. Bacteria photosynthesizing dissolved carbon dioxide in oceans
Explanation:
The levels of oxygen that we know nowadays in the atmosphere have not been like this in the more distant past. In order for the oxygen to start building up and change the composition of the atmosphere, something needed to happen, and that something was the cyanobaceteria. The cyanobacteria was able to perform photosynthesis, so it started to use the dissolved carbon for it, while in meantime it was releasing oxygen in the atmosphere. As the cynaobacteria was growing in numbers, over the course of millions of years, it managed to propel enough oxygen in the atmosphere so that the other organism can start developing, thus changing the world forever.
