If the need is for cleaner air, the problem might be that of reducing the dust discharge from power-plant attacks , or reducing the quantity of the irritants form automotive exhausts.

 
  • 问题比需求更具体。比如,如果你的需求是净化空气,那么,存在的问题可能是去减少电厂烟尘排放或汽车尾气排放量。
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