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.

本题要求计算OCEAN的年龄，而RIVER中的盐量和OCEAN有什么关系？

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。

文章是说：河流带给海洋盐。因此呢，如果海洋的年龄是可以被精确估算的，通过如下的方法：知道过去100年海洋盐分上升的幅度【速度】，就能估算海洋从没盐到现在盐分水平的时间了。没有盐分到现在水平的盐分的差值就是一个总的【路程】。【路程】/【速度】＝时间。

这里说要加强

E说海里的盐分不会消耗，意思盐分一直是增长的，没有损耗的。意思在于路程是只走一遍的，不会走了一点路，倒退一点，再走一点路，再倒退一点。这样的话时间会变长，因为速度本身是不变的。因为她是一个100年的平均值。