+1 for C

Conclusion:
Ichthyosaurs were deep divers.
Reason:
The outer shell of the bones was also porous in the ichthyosaur

(C) In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.
--> This weakens the argument as it gives us an example in which there were marine species who were not deep divers and still had outer shell porous bones.
_________________

I think D is wrong because the unavailability of evidences doesn't means that few special characteristic of whale were not shared by ichthyosaurs.

C is absolutely correct.

I am confused between C and D. I dont understand how to choose between the two. Why is D wrong?

Gmat Ninja, can you please explain why D is wrong and C is correct ? The reason I didnt choose C is that it has the word 'most'. That means ichthyosaur can be an exception.

I chose C. But i want to know why D is wrong.


Sent from my iPhone using GMAT Club Forum mobile app

Argument :
Deep Diving mammals --> Porous outer shell of the bones --> Swim back to surface after a deep dive

Conclusion :
Porous outer shell of the bones in ichthyosaur --> Deep Divers

So, the conclusion is that ichthyosaurs are Deep Divers and the evidence is that
they have porous outer shell of the bones.
In order to weaken this conclusion, we must hit the missing link between the evidence and the conclusion.

C says that most of the non deep divers have porous outer shell of the bones. Not a conclusion breaker but certainly
weakens the conclusion to a large extent.

D says In addition to porous outer shell of the bones, whales have some other characteristics that help them to deep dive.
Now,Firstly this choice talks about some characteristics that helps whales to deep dive (not the sole reason for them to become deep divers).
Now, irrespective of whether these characteristics are present in ichthyosaur, we cannot prove that ichthyosaur are not deep divers.
Can you say that since ichthyosaur does not have these characteristics, so they are not deep divers ? No,right ?
This choice is actually leaving you with a lot of confusion in mind.

It is important to attack the missing link and C exactly does that.

Hi gmatkillerdude
D says there is no clear evidence WHETHER the other attributes were present in icthyosaurs. D is doubtful about the presence of attributes. C is better in clearly stating a weakness of the argument.
D could have become a strong contender had it been written as below -

(D) In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there ARE SOME evidenceS THAT these were shared by ichthyosaurs.

Hope this helps.

In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous. This has the effect of making the bones light enough so that it is easy for the animals to swim back to the surface after a deep dive. The outer shell of the bones was also porous in the ichthyosaur, an extinct prehistoric marine reptile. We can conclude from this that ichthyosaurs were deep divers.

Type-weakens
Core- ichthyosaurs were deep divers because they have the same sort of porous bone structure that modern deep divers have, a structure that allows deep diving.
Pre-thinking- Maybe these porous bone structures helped ichthyosaurs in some other way- they could swim faster

(A) Some deep-diving marine species must surface after dives but do not have bones with porous outer shells.- Out of scope- it talks about species that do not have bones with porous outer shells
(B) In most modern marine reptile species, the outer shell of the bones is not porous.- Irrelevant
(C) In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.- Correct
(D) In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there is no clear evidence whether these were shared by ichthyosaurs.- incorrect; Just because there is no evidence, it does not mean the phenomenon doesn't exist.
(E) There is evidence that the bones of ichthyosaurs would have been light enough to allow surfacing even if the outer shells were not porous.- incorrect; it says that the ichthyosaurs probably didn't even need porous bones

Answer C
_________________

KarishmaB please could you help me with (A) and (E). While I understand why (C) is the answer I was not confident with eliminating (A) and (E)

Argument
1. The puter shell of the bones of modern deep-diving marine mammals is porous.
2. Porosity makes bones light, helping the animals to swim back to the surface after a deep dive.
3. Ichthyosaur, an extinct prehistoric marine reptile, had porous bones

Conclusion: Ichthyosaurs were deep divers.

Question: Weaken

We see a correlation between porous bones and deep divers. Based on this correlation we are making a conclusion.



Why (A) is not the answer: This choice talks about species that do not have porous outer shells WHILE we are concerned with species WITH porous outer shells. We could have a situation such that all porous outer shell species are deep divers while some non-porous outer shell species are deep divers. Thus if Ichthyosaurs have porous outer shells then the conclusion can be drawn.

Doubt with (A): The argument shows a correlation between porous outer shells and deep divers. But the conclusion depends on the causation i.e. porous outer shells --> Deep diving. So doesn't (A) showcase that porous outer shells are not needed for deep diving, thereby breaking the causation and thus showing that despite seeing the presence of porous outer shells one cannot conclude that the Ichthyosaurs were deep divers? Hence can't (A) be a weakener??



Doubt with (E): Porous out bone shells help make the bone light and thus help the (deep diving) species to float up. Seeing the presence of porous bone shells, the conclusion is drawn. But now (E) reduces the importance of the presence of porous bone shells. By showing that the ichthyosaurs already had light bones to surface up, this choice shows that porous bone shells were not needed for the species to be deep divers. Hence seeing porous bone shells (not a necessary factor) we cannot conclude that the ichthyosaurs were deep divers.

In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous. This has the effect of making the bones light enough so that it is easy for the animals to swim back to the surface after a deep dive. The outer shell of the bones was also porous in the ichthyosaur, an extinct prehistoric marine reptile. We can conclude from this that ichthyosaurs were deep divers.

Which one of the following, if true, most weakens the argument?

(A) Some deep-diving marine species must surface after dives but do not have bones with porous outer shells. - WRONG. First, we don't know whether 'some' includes ichthyosaurs or not. Even if it is, not having porous bones does not proves they were deep divers. Second, if ichthyosaurs are not included then the options falls flat - it is not at all near anywhere to core of the argument.
(B) In most modern marine reptile species, the outer shell of the bones is not porous. - WRONG. Irrelevant.
(C) In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous. - CORRECT. The word 'most' helps us know that ichthyosaurs could have been exception, thus weakening the argument.
(D) In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there is no clear evidence whether these were shared by ichthyosaurs. - WRONG. It slightly looks like to weaken the argument, however, it is not. If whales have other characteristics that help it deep dive, it does not mean ichthyosaurs could not have been deep divers. Had that characteristic been in ichthyosaurs, it would have made sense. At best this is irrelevant for it does not tell anything about ichthyosaurs' being deep divers.
(E) There is evidence that the bones of ichthyosaurs would have been light enough to allow surfacing even if the outer shells were not porous. - WRONG. What this gives is just another parameter similar to porosity but does not help at all, even minutely, that whether ichthyosaurs were deep divers or not.

Answer C.
_________________

Split the argument into premises and conclusion.

Premises:
In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous.
It helps them swim back to the surface after a deep dive.
The outer shell of the bones was also porous in the ichthyosaur.

Conclusion: ichthyosaurs were deep divers.

Based on the porous bones, we are concluding that Ichthy were deep divers. So we are saying that porous bones implies deep divers. We are saying that all porous bone marine animals were deep divers. We are not saying that all deep divers must have porous bones. We are saying that all animals with porous bones must be deep sea divers - that porous bones are sufficient to lead to deep sea diving. Because we are given porous bones, we are concluding deep sea diving.

We need to weaken this conclusion.

(A) Some deep-diving marine species must surface after dives but do not have bones with porous outer shells.

Doesn't impact our conclusion. We are not saying that all deep sea divers must have porous bones.
We are saying the opposite - that porous bones marine animals must be deep sea divers.

(B) In most modern marine reptile species, the outer shell of the bones is not porous.

We are talking about deep sea divers. Irrelevant.

(C) In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.

This tells us that porous bones are found in non deep sea divers too. So porous bones does not necessarily mean deep sea divers. This weakens our conclusion.

(D) In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there is no clear evidence whether these were shared by ichthyosaurs.

Irrelevant. We are establishing the connection between porous bones and deep sea diving only.

(E) There is evidence that the bones of ichthyosaurs would have been light enough to allow surfacing even if the outer shells were not porous.

Our argument does not say that porous bones are necessary for deep sea diving. It says that they are sufficient. It says that porous bones implies deep sea divers. Other things could also lead to surfacing after deep sea diving. This would weaken our conclusion if we were concluding that porous bones are necessary for deep sea diving. But we are not concluding that. Hence not correct.

Answer (C)
_________________