1. ____ is a source of clean energy. 2. Scientists believe ____ is changing the world's weather. 3. A ____ is a violent storm with strong winds. 4. ____ produce a lot of the world's oxygen. 5. ____ is energy from the sun. 6. In a ____, there is little or no water. 7. Air ____ is worse in cities than in the countryside. 8. A lot of rain can cause ____.

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