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ANSWERS 
1. The Ozone Layer 2. The Control of Pollution Act 3. Sulphur Dioxide 4. Biodegradable
5. Benzene 6. Alternative Energy 7. Catalytic Converters 8. Biological Control
9. Nitrate 10. Extinction 11. Aurora Borealis 12. Lose a Day
13. Prairie 14. Pacific 15. Crust 16. Skeletons of Tiny Creatures
17. The Atmosphere 18. A Type of Grass 19. Rice 20. 24 hours