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。