here assumption is just telling us that there is no alternate to what is stated in premise

CSASHISHPANDAY wrote:The optic nerves of identical twins are genetically identical. When only one of a set of identical twins is visually impaired, certain areas of the affected twin's optic nerve are atrophied whereas the corresponding areas of the other twin's optic nerve is unaffected. No such differences are found when neither twin is visually impaired. Therefore, this discovery proves definitively that blindness is caused by damage to the structure of the optic nerve.

The argument above depends upon which one of the following?

a.The optic nerve of a blind person is smaller than that of anyone not suffering from visual impairment.

b.The relative size of the optic nerve of visually impaired individuals is not the result of corrective treatments or medications used to treat visual impairment.

b.The optic nerve of a person with an identical twin is no larger, on average, than the optic nerve of a person who is not an identical twin.

d.When a set of identical twins are both visually impaired, their optic nerves are in the same relative condition.

e.Individuals who have an identical twin are no more likely to suffer from visual impairment than are those who do not.


I am unable to understand how B can be answer

Hi there!

I'm glad you asked about this problem, because it's a classic GMAT scenario. You chose a good one to post, because the lesson that it teaches is one applicable to a variety of similar problems.

For CR problems, always start with the question itself. This question asks for the assumption the argument relies on--what's the important, missing piece from the text?

Well, the text concludes that optic nerve damage is the cause of visual impairment.

What evidence does the author provide to support this claim? Quite a bit, actually. Twins are identical, and normally have identical optic nerves. However, when one suffers visual impairment, that twin will show atrophy to the optic nerve.

However, if we examine the evidence closely, we'll see a flaw in the reasoning. The evidence tells us that nerve damage and blindness are correlated. They happen at the same time. But the conclusion is that there is a causal relation, that the damage is the factor responsible for blindness.

The author's assumption is a very, very common GMAT assumption: that correlation implies causation. He assumes that the best and only explanation for the correlation he cites is that nerve damage causes visual impairment, and that no other explanation is more compelling.

And that prediction is what leads us to (B). A plausible explanation for the facts in the question stem is that somehow, the visual impairment is the cause of the optic nerve damage--that the author has cause and effect reversed! But the author does not even address this possibility. Therefore, it is correct to say that he assumes that this reversed causality is not true. And so, (B) is the correct answer.

Hope this helps, let me know if you have any more questions!

Thanks Now I got

very good problem!

Can someone please explain the argument.

This type of assumptions are called passive assumptions. In GMAT, to reach conclusion, when author assumes that A is causing B( A --> B ), then he assumes that A is the only cause for B. so by default he assumes that "there is no other thing C which is causing B". bold part in previous sentence is called passive assumption.

passive assumptions might look irrelevant or out of scope to the argument,because it generally contains new information not mentioned in the passage. so don't eliminate it, just because it has new information.

This is a defender type assumption question:-
Here, if A causes B, then nothing else can cause B.
OR If B is caused, then it would be A that caused it.
Option "B", fits the above explanation!!