What evidence suggests when oxygen gas first became a significant part of Earth's atmosphere?
A) multi-celled microorganisms that breathed oxygen
B) fossils of dinosaurs
C) molten rock from volcanoes that were rich in oxygen
D) rusty rocks indicating oxidized iron
E) the presence of water vapor in the atmosphere

Question

Quizplus · Accepted Answer

Rusty rocks indicate the presence of oxidized iron, which only forms in the presence of oxygen. Therefore, the presence of such rocks indicates that there was oxygen in the atmosphere at the time they were formed.

What Evidence Suggests When Oxygen Gas First Became a Significant