But noW it is realized that supplies of some of them are limited,and it is even possible to give a reasonable estimate of their“

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问题 But noW it is realized that supplies of some of them are limited,and it is even possible to give a reasonable estimate of their“expectation of life”,the time it will take to exhaust all known sources and reserves of these materials.

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答案在人们意识到,其中有些矿物质的蕴藏量是有限的,人们甚至还可以比较合理地估计出这些矿物质“可望存在多少年”,也就是说,经过若干年后,这些矿物的全部已知矿源和储量将消耗殆尽。

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