There is a heap of waste material in front of the laboratory. 實(shí)驗室前面有一堆廢料。
These methods are better than pollution control techniques that have to be applied after a waste material has become a potential pollutant. 這些方法比在廢料成為潛在污染物之后，才采用的污染控制技術(shù)更為優(yōu)勝。
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