请教各位NN:GWD16-29

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E，我能理解，但B好象也可以啊，如果不同时间河流溶解的盐不同，那么最近一百年测量的数据就没有代表性

BAt any given time, all the Earth’s rivers have about the same salt levels.

不对啦，没有SUPPOSE每条河都有相同的含盐度。

不过不明白A为什么不对，请指点！！