Concerned Parent: While I love the facilities and community that you have created at your school, I am troubled by the fact that only 75% of your teachers are certified by the department of education. I just cannot take the risk of a 25% likelihood my daughter will be taught by someone who is unqualified.

Which of the following is an assumption of the parent's argument?

A) Any teacher not certified by the department of education is unqualified.
B) The department of education is the entity best equipped to determine which teachers are qualified.
C) Unqualified teachers are less effective than qualified teachers.
D) The school in question has more uncertified teachers than any other school the parent is considering.
E) None of the teachers who are uncertified are in the process of becoming certified.
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VERITAS PREP OFFICIAL SOLUTION:

In this Assumption question, notice the wording gap between the two sentences. The first sentence says that 75% of teachers are certified (meaning that 25% are not certified), and the second changes "not certified" to "unqualified." Are certified and qualified necessarily the same thing?

If you see this gap, you should be ready for answer choice (A), which links certification to qualification by saying that anyone who is not certified is also not qualified. Note that if you hold (A) up to the Assumption Negation Technique, the opposite of (A) weakens the argument by showing, essentially, that some teachers who are not certified are still qualified. (A) is therefore correct.

Notice that none of the other answer choices deal with this gap, which is the major flaw in the argument. Among the other answer choices: (B) isn't a necessary assumption, as the department of education doesn't need to be the best organization at determining teacher qualification - as long as it can do so capably, then the argument doesn't fall apart. Choice (D) is similar: whether or not the school has the most unqualified teachers doesn't change the reason for the parent's skepticism. Choice (C) adds a wording gap of its own by bringing in a new term "effective" that doesn't exist within the argument. And choice (E) isn't directly applicable, as you do not know how long the certification process takes (is "in the process" something that would help the parent's daughter anytime soon?) or whether just one teacher is in the process (how much would that change the percentages that the parent is concerned about?).
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आत्मनॊ मोक्षार्थम् जगद्धिताय च

Resource: GMATPrep RCs With Solution

can some one pls explain why C is wrong