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1: Active solar heating system 2: Soil 3: Human 4: Community 5: Water 6: Environment 7: Lead 8: Condition 9: Feeder 10: Range 11: Process 12: Carbon 13: Contamination 14: Plant 15: Animal 16: Ecosystem 17: Electricity 18: Change 19: Conservation 20: Development (As of 00:40)