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~

sdcar2010

A rule of thumbs, if a qualifier used is too strong or restrictive, the answer containing the qualifier is unlikely the right answer for an assumption question.  It is too restrictive.

B and E contain such extreme qualifiers: At any given time, all . . .; None of the salts . . .

eliasfan

but it's not the reason to explain the question, it's just the way to chooce the answer to the question!
in fact ,I want to know why!

sdcar2010

The reason is simple.  An assumption has to be always right in order to enable the argument.  Do you think that it is reasonable to demand that "At any given time, all the Earth’s rivers have about the same salt levels" in order for the argument to keep going? That is an extreme condition to keep.  Why would an author walk that path when there are easier way to support the same argument?

The game of CR is to quickly devide the answer choices into losers and contenders, followed by picking the right choice.  As to the question at hand, the proper way is to use negation since the question stem asks for a necessary assumption.  If you negate A, the arugment falls apart.  Hence, A is the right answer.