请教一题GWD

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 ocean

正确答案是A，请问E为什么不对呢？谢谢

地球上的河流不断地把溶解的盐带入海洋。因此，通过测量过去几百年海洋盐的增加量，并确定这样的增加量需要花费多少世纪，我们可以在海洋初始状态无盐的假设下精确估计地球海洋的年龄。

上面的论证的假设是什么？

a过去几百年内河流带入海洋的溶解盐并不是异常的多

b在任何时间，地球各条河流都有相同的盐分水平

c有盐直接从洋盆渗入海洋

e没有更好的基于盐分来估计地球海洋年龄的方法

e河流带入海洋的盐中没有被海洋中的生物活动消耗

a正确，海洋年龄（百年）=【现在的盐分水平-初始盐分水平0】/(盐分几百年的增加量），分母过大，得出的数据肯定有很大偏差，例如现在盐分水平为a，那么几百年盐分增加量要足够小，0.00。。。a，才能测算，如果这一百年的增加量是0.1a，那么海洋年龄只有1千年，显然这种计算方法是错误的 

b无关，因为考虑的是海洋盐分的增加量，与河流无关，第一句话只是说明盐分进入的一个途径

c无关，因为考虑的是海洋盐分的净增加量就可以测量

d跑题

e无关，生物活动只是盐分流出的一个途径，无所谓

个人这道题出得有点邪乎，那个therefore没有意义，我读题干时，觉得应该有个正确选项说，海洋盐分增加速率始终保持在一个稳定的水平