In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous. This has the effect of ...

meisen on May 31, 2018


I chose A, why is C correct?

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Christopher on June 6, 2018

@meisen, this is a weakening question, so we're looking for information that could derail the logic used in the question. The argument is that since the porous outer shell of deep-diving mammals's bones allows those mammals to resurface after a deep dive, any animal that has bones with a porous outer shell must be a deep-diver. Essentially: BPOS ==> DD and contrapositively Not DD ==> Not BPOS.

So once you've narrowed that down, you're looking for might weaken that logic.

(A) says that animals do not NEED to have bones with a porous outer shell to be deep divers, but that doesn't impact the logic. The question isn't arguing that all animals that deep-dive have bones with a porous outer shell but rather that all animals that have bones with a porous outer shell are deep divers.

(C) however, provides an alternate explanation by saying that most marine reptiles are not deep divers yet still have bones with a porous outer shell. This provides an alternate explanation and throws the logic presented in the question into do.

This question hinges on the author's mixing of necessary and sufficient conditions. Deep Divers have Bones with Porous Outer Shells: DD ==> BPOS, and if an animal does not have Bones with Porous Outer Shells it is not a Deep Diver: Not BPOS ==> Not DD. However, to say that to have Bones with Porous Outer Shells means that an animal is a Deep Diver gets the logical cart before the horse, and answer (C) points that out.

Margaret on September 4, 2018

Well said, thank you very much for explaining!

Steph on March 16, 2019

why is D incorrect?

Bhavraj on July 3, 2019

I can see why C is correct but am having trouble getting rid of D. Can someone explain to me why D does not weaken the argument?

Ravi on July 4, 2019

@Margaret, happy Christopher's explanation was helpful!

@Steph and @noname,

Let's take a look at (D).

(D) says, "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

The problem with (D) is that we do not know for certain whether the
dinosaurs possessed these characteristics, so we cannot use (D) to
argue against the dinosaurs' deep diving capabilities. For this
reason, we can get rid of (D).

Does this make sense? Let us know if you have any other questions!