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.

大侠，A为什么不行呢？A中条件如不满足，这一百年的盐含量高许多，那我们由此算出的海洋年龄就不正确了。值应该比实际小许多。

个人认为B和E看起来都有道理，但答案中用词很绝对。动不动就是any, all, none。这种答案未免太绝对。所以应该排除。

答案就是a

e是不对的，大家要看清楚salts,表明各种盐，比如钾盐，钠盐什么的， 一种或几种用完并不影响计算结果， 只要有一种不变就行。

再分析第一项，地球的年龄=海水中的含盐量/过去一百年的流入盐量，当然再乘以一百 得出， 所以假如过去一百年的盐流入量非常之高，那么计算的地球年龄就大大短于其真实年龄。