#21 - In modern deep-diving marine mammals, such as whales,

[heartofsunshine]

Hi there,

I chose A for this one instead of C. I was running low on time and so when I read A I felt pretty confident and went ahead and moved on. Now reading through the answers I see why C is correct, but still do not see why A is incorrect. C is correct because, if non deep diving marine reptiles have porous bones, and the only evidence we have from the ichthyosaur is the porous bones, this would not be evidence to determine if it was a deep diving animal.

In A though, if some marine animals do not have porous bones, but they are deep divers, I see this also contradicts. Because it means that deep diving marine mammals can have either porous or non porous bones. Therefore once again if the only evidence we have is the porous bones, it's not enough to determine if it was a deep diving mammal or not.

Thank you!

[James Finch]

Hi Heart,

A big clue with (A) is a the "some," which is incredibly vague and can only really weaken conclusions that are trying to prove that "none" do something, or in this case all things with one characteristic also have another characteristic. This could potentially work in this case, as the conclusion is 100% certain that Porous Outer Shell Deep Diver, but then (A) also subtly changes the scope of the effect from "deep dive" to "dive" while, to its credit, maintains the correct cause. Because this answer choice doesn't give us any information about "deep divers," only "divers," we can't use it to attack a conclusion about the more specific "deep divers."

Hope this clears things up!

[gstf]

How is D not also correct? It seems to cut into the argument that poruos bones=deep diver. It opens the possibility that poruos bones alone are not enough to be deep divers, which cuts into the argument that the ichthyosaur was probably a deep diver purely on the basis of its porous outer shell.

[KelseyWoods]

Hi GSTF!

In this question, we're trying to weaken the argument that ichthyosaur was a deep diver based solely on its porous bones. The argument essentially tells us that having porous bones is sufficient to prove that ichthyosaur was a deep diver. We're looking for an answer choice that shows that porous bones are not enough to prove that ichthyosaur was a deep diver, which you rightfully point out.

Answer choice (D) doesn't accomplish this task for us, though. It tells us that whales have some characteristics that are suited to deep diving that ichthyosaurs probably didn't have. But this doesn't truly weaken the argument that porous bones are enough to prove ichthyosaur was a deep diver. It just lets us know that whales have other characteristics suited for deep diving. Ichthyosaurs could have had different characteristics that are well suited for deep diving. But in both whales and ichthyosaurs and any other animals, porous bones alone could still be enough to prove that they are deep divers. The additional characteristics--or lack there of--are irrelevant as to whether porous bones alone are enough to prove that ichthyosaur was a deep diver.

Hope this helps!

Best,
Kelsey

[Tarte au chocolate]

can u explain why C is right tho?

[Jeremy Press]

Hi Tarte,

The reason answer choice C is correct is because it shows that the only physical feature we know about the ichthyosaur, that it has porous bones, is not enough by itself to tell us they were deep divers. This is because answer choice C shows that a significant percentage of similar animals (marine reptiles, both modern and prehistoic) had the same physical feature (the porous bones) but were not deep divers. Since answer choice C makes us doubt whether porous bones necessarily indicate deep diving, it severs the link between the premise and conclusion. That's a great way to weaken an argument!

I hope this helps!

[teezoTD]

I chose C at first and then switched to E and I couldn't see how it was incorrect. Is it because E is weakening the premise that the ichthyosaur had porous outer shells instead of weakening the argument that they were deep divers?

Let me know if that's the right reasoning to eliminate answer choices like E on future weakening questions. Thanks.

[Jeff Wren]

Hi teezo,

We're trying to weaken the conclusion that ichthyosaurs were deep divers based on the fact that they had porous bones (like other deep-diving marine mammals).

Answer E mentions that the bones of ichthyosaurs were light enough that they didn't need to be porous. This doesn't weaken the argument at all. If the bones were already light, that's great for the ichthyosaurs and their ability to deep dive. If that were the case, having the porous bones (which we know that they did) could just be an extra advantage in deep diving, just making it that much easier.

In other words, this answer would probably strengthen the argument if anything. Any answer that increases the odds that ichthyosaurs were deep divers, or could have been deep divers, helps the argument.

If the answer had given a reason why their bones were otherwise too dense so that the porous outer shell wouldn't be enough to allow surfacing (or given some other reason why they wouldn't be able to deep dive), that would weaken the argument.