The Earth's atmosphere Choose one: A. acquires its carbon dioxide from photosynthesis by green plants. B. became more oxygen-rich during the Proterozoic, after the evolution of photosynthetic organisms in the Archean. C. began as nitrogen and oxygen, left over from the solar nebula. D. gets warmer as its carbon dioxide concentration goes down.

The amount of oxygen in the initial stage was significantly less. The ocean initially was comprised of a certain amount of oxygen, which was present in the dissolved state, and some of it was trapped in the iron-rich rocks commonly known as the Banded Iron Formation (BIF).

With the growing abundance of cyanobacteria, stromatolites, and other phytoplanktons and marine plants, the rate of photosynthesis eventually increased during the Precambrian period. Also, the land plants were developed during this time, and due to this increasing photosynthesis rate, the amount of oxygen in the earth's atmosphere gradually increased, thereby supporting a large number of living organisms.

Thus, the earth became more oxygen-rich during the Proterozoic age (2500 to 540 million years back).

Hence, the correct answer is option (B).