Question

Oxygen is returned to the atmosphere mainly by

• A. burning of fossil fuels
• B. respiration
• C. photosynthesis
• D. Fungi

Answer 1

The correct option is C

photosynthesis

The correct option is C.

Explanation of the correct option:

1. Photosynthesis is the primary means of returning oxygen to the atmosphere.
2. According to Earth's history, cyanobacteria were the first to introduce oxygen gas into the atmosphere through photosynthesis.
3. There was no life on Earth around 4.6 billion years ago because the atmosphere was devoid of oxygen.
4. All green plants release oxygen into the atmosphere as a by-product of photosynthesis.
5. For respiration, all aerobic organisms utilize free oxygen.
6. Carbon dioxide is exhaled by animals and returned to the atmosphere, where plants use it during photosynthesis.
7. As a result, the amount of oxygen in the atmosphere is now balanced.

Explanation of the incorrect options:

Option A:

1. CO₂ is released into the atmosphere as a result of the combustion of fossil fuels.
2. Indeed, not the oxygen, and therefore this option is incorrect.

Option B:

1. Respiration is the process of breaking down glucose in our bodies, either with or without the use of oxygen, to produce energy.
2. No oxygen is released into the atmosphere during respiration.
3. Therefore, this option is also incorrect.

Option D:

1. Fungi are eukaryotic, multicellular organisms.
2. They are saprophytes, which means they eat dead organic matter.
3. As a result, we can say that photosynthesis is the primary source of oxygen returned to the atmosphere.
4. Therefore, this option is also incorrect.

Final answer: Oxygen is produced in the atmosphere mainly by photosynthesis.