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题库 / Prep2012-CR-73
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?
The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
At any given time, all the Earth's rivers have about the same salt levels.
There are salts that leach into the Earth's oceans directly from the ocean floor.
There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.
E我是这么理解的：没有一种流入海洋的盐被生物活动用光。
注意“None of”，其取非是“At least some of”，use up“用光了”而不是用了一部分，每年流进来都被用光了的那些种类的盐应该不能用来检测的吧。所以这句话取非是：至少有一些流入海洋的盐被生物活动用光。那还能通过另外没被用光的盐来推测的嘛，所以不能削弱。
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