But now it is realized that supplies of some of them are limited, and it is even possibleB
ut 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.
【参考答案】可是现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“预期寿命”也就是说经过若干年后......
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