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Here are the corrections: 20. The mistake is "Burning" should be "Burn". The correct sentence is: "Burnt in open fires, rubbish and organic waste produce a great amount of black carbon." 21. The mistake is "Encouraged" should be "Encouraged by". However, the correct sentence would be: "Encouraged by students' interest in climate change, the Green Club members organised many activities to raise awareness of the issue." (Alternatively, the sentence could be "Encouraging students to take an interest in climate change, the Green Club members organised many activities to raise awareness of the issue.") However in this case there is no mistake. 22. The mistake is "cut/cutting" should be "cut". The correct sentence is: "When cut down, trees release a lot of carbon dioxide into the atmosphere." 23. The mistake is "Done" should be "Having done". However, the correct sentence would be: "Having done research on climate change, Mike became interested in environmental studies." 24. The mistake is "Using" should be "Used". The correct sentence is: "Used widely for cooking and heating, sunlight and wind can reduce use of fossil fuels."