The molecular oxygen in Earth's present atmosphere was most probably produced:
A) at the formation of Earth and has always been present.
B) from volcanic eruptions as the primitive Earth cooled down.
C) from volcanoes, by out-gassing.
D) by biological activity such as photosynthesis from living things.

Question

Quizplus · Accepted Answer

Molecular oxygen is produced by photosynthesis from living things such as plants, algae, and cyanobacteria. There is no evidence of significant levels of oxygen in the Earth's atmosphere prior to the appearance of these organisms.

The Molecular Oxygen in Earth's Present Atmosphere Was Most Probably