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.  

the answer is E,but B is reasonalbe as well.

楼上说的是，我脑子不清楚了。

应该根据海里盐量算出年龄，只要总含盐量准确就可以，跟河里的含盐量没有关系。

我手上的蓝皮书里，这道题的答案是 A.。其实我对 A 选项的意思还有一点不太明白的，就是其中的红色字体部分。这句话的意思是不是：在过去的几百年里，河流中溶解的盐沉积在地球的海洋中的量没有异乎寻常的大？翻译过来总觉得有点别扭。