Self-study
Support In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous. ████ ███ ███ ██████ ██ ██████ ███ █████ █████ ██████ ██ ████ ██ ██ ████ ███ ███ ███████ ██ ████ ████ ██ ███ ███████ █████ █ ████ █████ ███ █████ █████ ██ ███ █████ ███ ████ ██████ ██ ███ ████████████ ██ ███████ ███████████ ██████ ████████ ██ ███ ████████ ████ ████ ████ ████████████ ████ ████ ███████
Summarize Argument: Phenomenon-Hypothesis
The author hypothesizes that ichthyosaurs—extinct marine reptiles—were deep divers. This is because ichthyosaurs share a feature with deep-diving mammals today: a porous outer bone shell, which helps deep-diving mammals surface after deep dives.
Based on an observed similarity in bone structure between ichthyosaurs and deep-diving mammals, the author hypothesizes that ichthyosaurs also shared a behavioral similarity with deep-diving mammals. In other words, the author draws an analogy between the two types of animals. To weaken, we can attack the analogy by finding relevant dissimilarities. We can also attack the phenomenon-hypothesis argument by presenting an alternative explanation or contrary evidence.
21.
Which one of the following, ██ █████ ████ ███████ ███ █████████
a
Some deep-diving marine ███████ ████ ███████ █████ █████ ███ ██ ███ ████ █████ ████ ██████ █████ ███████
The argument doesn't rely on a claim that a porous outer bone shell is necessary for deep dives, so refuting that doesn't weaken. If this were reversed to say that some animals with a porous outer bone shell were not deep divers, that could weaken, but as is it doesn't harm the argument.
b
In most modern ██████ ███████ ████████ ███ █████ █████ ██ ███ █████ ██ ███ ███████
We have no idea if those reptiles are deep-divers. Even if we knew that they were, this wouldn't weaken for the same reason that (A) doesn't: the author doesn't assume that porous outer bone shells are necessary for deep-diving, so undermining that idea doesn't make a difference.
c
In most modern ███ ███████████ ██████ ███████ ███████ ████ ███ ███ ████ ███████ ███ █████ █████ ██ ███ █████ ██ ███████
This provides contrary evidence which indicates an alternative explanation for the ichthyosaurs' bone structure: it could be for the same reason that most non-deep-diving marine reptiles have porous outer bone shells. This effectively undermines the argument.
d
In addition to ███ ██████ █████ ██████ ██ █████ ██████ ██████ ████ ██ █████ ████ ███████████████ ██████ ██ ████ ██████ ███ █████ █████ ██ ██ █████ ████████ ███████ █████ ████ ██████ ██ █████████████
All this says is that we don't have additional information; it doesn't say that ichthyosaurs didn't share these characteristics, just that we don't know. Sure, this doesn't strengthen the analogy, but it doesn't weaken either. It does nothing at all.
e
There is evidence ████ ███ █████ ██ ████████████ █████ ████ ████ █████ ██████ ██ █████ █████████ ████ ██ ███ █████ ██████ ████ ███ ███████
This tells us that ichthyosaurs could’ve been deep divers even without porous outer bone shells. But we're trying to weaken the claim that ichthyosaurs were deep divers at all, so finding more ways for ichthyosaurs to be deep divers doesn't help us.