a gradual decline逐渐衰落[例句]Industrialization, first in Europe and later in America, led to a gradual decline in the use of windmills. 工业化的出现导致了风车的使用逐渐减少，先是在欧洲，后来在美国。
你是指产品的质量下降，还是指物理上的物体质量的下降啊??(产品的)质量下降: the quality gets lowerthe quality declinesthe quality descends(物体的)质量下降：the mass decreasesthe mass is cut downthe mass is reduced
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