The molecular oxygen in Earth's present atmosphere was MOST likely 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 outgassing.
D) by biological activity such as photosynthesis from green plants.

Question

Quizplus · Accepted Answer

The molecular oxygen in Earth's atmosphere primarily comes from biological processes, especially photosynthesis performed by green plants, algae, and cyanobacteria, which convert carbon dioxide and water into oxygen and glucose using sunlight.

The Molecular Oxygen in Earth's Present Atmosphere Was MOST Likely