GWD-16

QingHuang

Why the anwser is E, not A?  Thanks for your explaination!

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?

 

1. The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
2. At any given time, all the Earth’s rivers have about the same salt levels.
3. There are salts that leach into the Earth’s oceans directly from the ocean floor.
4. There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
5. None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.

   Answer: E

QingHuang

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simonshen

我觉得这题的题目不是很严谨（也可能我理解错误？）。先说下我的分析，大家可以讨论下。地球上的河流常年都会把溶解的盐带到海里，所以把过去100年里海洋中盐份的增加量算出来（假设这个变量是S+），然后算出现在海洋里总的含盐量（ST）和理论上最初海洋里没有盐的状态（0），然后算下总共花了多少世纪达到这个含盐度（ST），就可以算出地球的存在时间(TT)。这个意思好像是

(ST - 0) / S+ = TT

E是肯定必须做的假设，如果有很多盐在生物活动中消耗了，那肯定这个等式就不准确了，因为那样ST就不是海洋这么多年实际的积累下来的盐量了。

另外一个我认为必须要做的假设是S+（即过去100年内河流代入海洋的盐量）是多年来的平均数，否则无法通过这个题目所说的方法得出地球的存在时间。A是保证过去100年带来的盐份不是unusually large,但其实关键应该是要保证过去100年带来的盐量是常年的平均值，那才是一个必须做的假设了。如果过去100年带来的含盐量是unusually large,但是以前一直以来每世纪的这个量也都是一样large，那题目还是成立的。关键就是unusually 这个词让人觉得过去100年的量是比以往都要大了，大得unusual了，所以A也有一定道理了，于是乎我就觉得这题目出得不是太严谨了。但是相比E是更确凿无疑的。整个题目其实最关键的假设是过去100年的量是平均数，但没有这个选项。

以上是我愚见哈,仅供参考!