Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth? 1. Volcanic action 2. Respiration 3. Photosynthesis 4. Decay of organic matter. Select the correct answer using the code given below.

1. Volcanic action — Volcanoes release CO2 from Earth’s interior into the atmosphere. Correct.

2. Respiration — Living organisms respire and emit carbon dioxide as a by-product. Correct.

3. Photosynthesis — Green plants remove CO2 from the atmosphere to make food; they do not add it. Incorrect.

4. Decay of organic matter — Decomposition releases CO2 (and sometimes methane under anaerobic conditions) back into the carbon cycle. Correct.

So, 1, 2 and 4 only are correct.