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.作者: bobchowz 时间: 2012-2-27 19:08
assumption的题一般在最后都排除完以后剩下的两三个都很有吸引力的选项里使用取非法。你把E取非后就变成At least some of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans. 既然是some，它对the resulting increase in salt levels in the oceans over the past hundred years 是不会有很大影响的。
相对而言，A是最直接的assumption，取非A以后变成The quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusuallylarge during the past hundred years. 原来的premise说的是constantly，这显然是weaken了原premise，因此使得conclusion falls apart。那么A就是正确答案。
其实不用取非法这题就直接能看出来，因为premise说的是constantly，而A说的是not unusually large，等于是加强了premise。作者: quasimodov 时间: 2012-2-28 06:48
这里不是在追究大海的溶解盐的问题，这题目是说通过测量大海的the increase in salt levels ，并且通过从大海没有salt level 到有salt level 的这个增加量来算花了多少时间形成，去判断 大海的最大年龄是多少，也就是大海在多久以前就开始形成了，所以说这和 the Earth’s rivers （就是大海为形成的前身）有没有一样的salt level 或者你说的dissolved salts 都是没有关系的，rivers 不管是否一不一样，判断ocean的最大年龄只和它自身的salt level 花了多少时间形成有关，和rivers 一不一样无关，只是rivers在这里给ocean带来的 salt的增加量。作者: uniquenellier 时间: 2012-2-28 19:01
简单讲：“生物活动消耗掉盐”并一定不影响 the increase in salt levels。如果，生物活动很稳定，盐的消耗量也基本没有变化呢？
so
选项E取非之后argument依旧能够成立。
the answer is a, you see,E None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans. ---> none salts are ued up by biological can not identify sth. if you insist, I think you just know no salt here consumed by biological. but it is nonsense.作者: Gracespa 时间: 2012-3-1 18:58
这道题是挺难的，我第一次也是纠结于A和E，但后来想了后明白了到底怎么回事。。。先说B，大家质疑的最多的，At any given time, all the Earth’s rivers have about the same salt levels.这里的rivers是指同一个given time的，所以要求rivers都有一样的盐浓度是没有必要的，你们想的应该是不同时期的rivers盐浓度相同，这个有关，也有影响，但不是GMAC的人的想法。。。再说E，其实这个也很有迷惑性，但是其实used up一看我就觉得有问题，其实每个时期进入量by rivers和消耗量by biological activities的差值才是salt level的increase，有后面这个也是不影响各个时期的increase的，只要各个时期这个量近似相等，当然本题没有探讨这个问题，E只是提到了消耗量，但并不会因为没有这个assumption就没有了对应的conclusion。不知道我讲明白没有，大家可以借鉴下。。供以后大家参考。。
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