GWD9-29

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.
答案是A，这是什么意思啊~~为什么E不对呢？我觉得E很有道理啊~~~储存进海洋的盐被由于一些生物原因所导致的现象用尽了~也就不能根据这个海平面盐的变化值来推断海洋的最大年龄了嘛~~