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

Non progredi est regredi

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.

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.

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.

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
_________________

When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. - Henry Ford
The Moment You Think About Giving Up, Think Of The Reason Why You Held On So Long
+1 Kudos if you find this post helpful