Natural pollutants refer to environmental pollutants caused by natural reasons, as opposed to artificial pollutants. Natural pollutants exceeding a certain amount may also cause environmental pollution. Which of the following are not natural pollutants? ['Defoliation in the forest, etc. releases a large amount of carbon dioxide through bacterial decomposition.', 'Visibility decreases due to heavy fog weather, and the burning of straw on the ground leads to long-term maintenance of foggy weather.', "A 1-hour space comet hailstorm hit the earth 13,000 years ago, seriously polluting the earth's air and causing the earth to enter the mini-glacial period.", 'The volcanic eruption of Kamchatka Peninsula has caused serious pollution to the atmosphere.']