read the following passage and answer the following questions.
Molecular and chemical evidences indicate that oxygenic photosynthesis first appeared on Earth several hundred million years before significant levels of O₂ appeared in the atmosphere. However, the O₂ that cyanobacteria produced could not accumulate in the atmosphere because it reacted spontaneously with the reduced iron minerals in the oceans to make iron oxides. By 2.4 billion years ago, O₂ levels had risen to one part per million. Iron oxides are poorly soluble in water and would have precipitated in the oceans, settling down onto seafloor, and forming the sedimentary structures with deposits of iron and silica-rich materials. During this span of Earth history, lasting more than 1.5 billion years, the presence of precipitating iron minerals could have caused the oceans to appear brown, or black, or even red rather than the blue colour we know today. Only after the abundant Fe²⁺ on Earth was oxidized, could O₂ begin to accumulate in the atmosphere, and not until 600–900 million years ago did atmospheric O₂ reach present-day levels. As O₂ accumulated on Earth, the atmosphere gradually changed from anoxic to oxic.

Based on the information given in the passage, what could be the most likely level of atmospheric O₂ 2.6 billion years ago?
Options:

Correct option is A

From the passage:
· Oxygen photosynthesis first appeared on Earth several hundred million years before significant levels of O₂ appeared in the atmosphere.
· The passage indicates that oxygen reacted with reduced iron minerals, forming iron oxides, and hence atmospheric oxygen levels remained very low.
· Around 2.4 billion years ago, oxygen levels rose to one part per million (1 ppm). Therefore, prior to this period, around 2.6 billion years ago, the oxygen concentration would be expected to be lower than 1 ppm, i.e., approximately 0.5 ppm.
Thus, 0.5 ppm is the most plausible estimate for the atmospheric oxygen level 2.6 billion years ago.
Key Takeaways (Information Booster): 1. Oxygen Photosynthesis:
· Cyanobacteria are believed to be the first organisms capable of oxygenic photosynthesis.
2. Iron Oxide Formation:
· Oxygen produced by cyanobacteria reacted with iron in oceans, forming precipitates of iron oxides.
3. Great Oxidation Event (GOE):
· Around 2.4 billion years ago, atmospheric O₂ began accumulating significantly.
4. Transition Period:
· Before the GOE, oxygen concentrations in the atmosphere were extremely low, remaining below 1 ppm.
5. Impact on Oceans:
· Precipitation of iron minerals colored the oceans brown, black, or red due to the abundance of Fe²⁺ and Fe³⁺ ions.