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nightblade354
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Joined: 31 Jul 2017
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Status:He came. He saw. He conquered. -- Going to Business School -- Corruptus in Extremis
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Schools: Darden '25 (M)
Expert reply
16 May 2019, 05:20
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D
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
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68% (01:48) correct
32%
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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
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
17 May 2019, 06:13
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Quote:
The right answer here should be D. As with any question on the GMAT, the first task is to identify what the question wants us to do. In this case, it is a structure based CR question, in which we are being asked to mimic the given argument.
In order to do this well, it helps if we can classify the argument in terms of x,y,z. This allows us to ignore the specifics of the situation and focus on the argumentative logic alone. For this question, the logic is:
X (normal 3 month babies) have Y (no more reflexes)
This thing doesn't have Y (still reflexes), hence it is not X
A - X causes Y, this is not X, hence it wont cause Y. This is not the same logic. OUT
B - X (Apes) have Y (inability to talk), this thing is X (Suzy is ape), hence it will also have Y (she cant talk). This is not the same logic either. OUT
C - X (humans) have Y (social skills), this thing has Y (social skills) and is therefore.... Z? (normal?). This is vastly different logic. OUT
D - X (opossums) have Y (abdominal pouches), this thing doesn't have Y (this animal has no pouch), hence it is not X (opposum). This logic is an exact match and therefore CORRECT
E - X (some trees) have Y (shedding pattern), this thing doesn't have Y (not shed), therefore it is not... Z? (normal?). Like in C, this logic is far from the logic used. OUT
One final thing to note: Options C and E used the word "normal" since the original stimulus used it as well. This is a trap. When you are asked to understand the structure of the argument, remember that the GMAT can use similar sounding content to lead you down the wrong path. Don't fall for it!
- Matoo
12 Apr 2021, 03:00
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nightblade354
OFFICIAL EXPLANATION
This is a straightforward argument by contraposition. The sentence "Normal full-term babies are all born with certain instinctive reflexes that disappear by the age of two months" can be reworded as an if-then statement: "If a baby is normal and full-term, then it will not exhibit certain instinctive reflexes after
the age of two months." This in turn can be symbolized as follows:
N—>~R
(Where N stands for "normal full-term" and ~R stands for "not having the reflexes.") Next, the sentence "because this three-month-old baby exhibits these reflexes, this baby is not a normal full-term baby" can be reworded as "If a three-month-old baby still exhibits certain instinctive reflexes, then it is not a normal full-term baby." This in turn can be symbolized as follows:
R—>~N
Clearly, this is the contrapositive of the previous diagram, so we are looking for an answer-choice that presents an if-then statement and then forms its contrapositive. Now in choice (D), the phrase "opossums have abdominal pouches" contains an embedded if-then statement: "If an animal is an opossum, then it has an abdominal pouch." Which can be symbolized as
O—>P
Choice (D) then states that this animal does not have a pouch and therefore is not an opossum. This can be symbolized as
~P—>~O
Clearly, this is the contrapositive of the previous diagram. Hence the answer is (D).
