Question

Scientists infer that oxygen in Earth’s atmosphere

did not exist in large quantities until after
(1) the first multicellular, soft-bodied marine
organisms appeared on Earth
(2) the initial opening of the Atlantic Ocean
(3) the first sexually reproducing organisms
appeared on Earth
(4) photosynthetic cyanobacteria evolved in
Earth’s oceans

Answer 1

Photosynthesis is the process of converting carbon dioxide into oxygen. Oxygen didn't really exist in the atmosphere until photosynthetic organisms appeared and evolved.