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CAT Critical Reasoning Practice question with Solution 41

Normal full-term babies are all born with certain instinctive reflexes that disappear by the age of two months. Because this three-month-old baby exhibits these reflexes, this baby is not a normal full-term baby.

Which one of the following has a logical structure most like that of the argument above?

[A]. Because carbon dioxide turns limewater milky and this gas is oxygen, it will not turn limewater milky.
[B]. Because no ape can talk and Suzy is an ape, Suzy cannot talk.
[C]. Because humans are social animals and Henry is sociable, Henry is normal.
[D]. Because opossums have abdominal pouches and this animal lacks any such pouch, this animal is not an opossum.
[E]. Because some types of trees shed their leaves annually and this tree has not shed its leaves, it is not normal.
Answer: D

We can symbolize the first sentence: “all X (normal full-term babies) are Y (born with certain reflexes that disappear by the age of two months).” Then the argument runs, “NOTY (this baby’s reflexes have not disappeared by the end of two months), therefore NOT-X (this baby isn’t a full-term baby).” Simple enough: all X are Y; NOT-Y, therefore NOT-X.

The parallel is (D): all X (“Opossums,” or “All opossums”) are Y (possess abdominal pouches). NOT-Y (this animal does not possess an abdominal pouch), therefore NOT-X (this animal isn’t an opossum).

(A) runs: all X (Carbon dioxide or “All carbon dioxide”) is Y (turns limewater milky). NOT-X (this gas is NOT carbon dioxide; i.e. it’s oxygen), therefore NOT-Y (it won’t turn limewater milky). Invalid and not parallel.

(B) runs: all X (All apes) are Y (are unable to talk). X (Suzy is an ape), therefore Y (Suzy can’t talk). Again, not parallel.

(C) is a casual argument that doesn’t even adapt well to symbolization. We’re never told that Henry is human—we have to assume that. Moreover, the conclusion introduces the concept of “normalcy” in general, whereas in the stimulus normalcy is used in regard to a very specific respect (a normal full-term baby).

(E) begins talking about “some” trees instead of “trees” or “all trees,” so right away you know it doesn’t parallel the stimulus. Moreover, (E) is a casual argument that fudges its terms, going from “sheds its leaves annually” to “has not shed its leaves.” When? So far? Ever? There’s no such uncertainty in the stimulus.

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