The Earth’s rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth’s oceans can be accurately estimated.
Which of the following is an assumption on which the argument depends?
A. The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
B. At any given time, all the Earth’s rivers have about the same salt levels.
C. There are salts that leach into the Earth’s oceans directly from the ocean floor.
D. There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
E. None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
而且E选项出现了一个biological activity，原文没有提及的词。
即使biological activity会use up salt，这也是个一般规律，一直会发生，不会对测量造成很大的影响。再退一步，如果salt is totally used up by biological activity，那就根本不存在题目描述的可以根据salt来测量age of ocean。作者: finnlijing 时间: 2012-5-11 06:29
补充一下。
E选项，none取非是some。
some salt的变动范围就很大了，如果只是万分之一或者亿分之一的salt被use up，对估计age of ocean是不会造成影响的。
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