求讲解~~

arieslee0410

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.
E.    None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
其实我觉得这三个答案都挺对的……
谁给我讲讲为什么就选A呢？
谢谢~