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            | Reference Key |  |   
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                  | 1. The sandstorm is a horrible phenomenon. The 
                    clouds of dust are so heavy that you can hardly breathe. It 
                    is so dark that it may cause traffic accidents. 2. Droughts result from lack of water, which is essential 
                    for plants and animals. The failure of crops leads to food 
                    shortage, and sometimes even famine.
 3. The flood washes away houses, destroys fields, and causes 
                    life and property losses. People may be left homeless and 
                    suffer diseases.
 4. The increase of disasters is mainly caused by human activities. 
                    With the growth of population, we need more resources from 
                    the Earth. The deforestation, over-cultivation, together with 
                    other factors, lead to the imbalance of the ecosystem. And 
                    this in turn causes more natural disasters.
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