GWD-12-29

Q29:

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.

答案是Ａ．我的想法是即使如Ａ所说，我们还是可以计算的。　关键是ｄｉｓｓｏｌｖｅｄ的量ｏｖｅｒ　ｔｈｅ　ｙｅａｒｓ　大体较平均即可，　就算量每年都大。

相比之下，我选了ｅ，　尽管这个也不是很对。望指教！！

请看MAKI的置顶贴GWD什么逻辑提问的.

这题以前讨论过。对A取非，即quantities of dissolved salts deposited by rivers in the Earth's oceans have been unusually large during the past hundred years. 那么以此为标准计算出来的年龄会比实际的小，就不能说是maximum的。而E中所说的微生物消耗盐，虽然也会影响实际年龄的准确性，但得出的结果仍然可以说是the maximum age of the Earth’s oceans。