先看看带介词的区别:
Their profits have fallen by 30 percent. 他们的利润下降了百分之三十。
Their profits have fallen to 30 percent. 他们的利润下降到百分之三十。
Their profits have fallen 30 percent.
如果去掉介词,那么,到底是 “下降了” 还是 “下降到”,不得而知,而且造成歧义。所以,无论介词 by 还是 to,均不可省略!
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