Correct answer: A

(A): Correct. The math flows as follows, taking 100 contributors:

70 contributors by ads
30 not by ads
At least 2x more contributors from ads (2/3 of the 3/3) by adding the months = 70x3 = +140
New total = 240 (2.4x original)

(B): Incorrect. (B) is out of scope; we do not know how the cash position of wheat farmers their propensity to contribute in the fall or by what amount they will increase contributions (i.e., will the number of contributors double?).

(C): Incorrect. (C) opposes the president's conclusion, suggesting that contributors will not increase by a large amount if ads are increased.

(D): Incorrect. (D) merely restates an established fact: 70% of contributors come from the ads.

(E): Incorrect. (E) is out of scope by referring only to the number of readers, not how those readers will contribute to the PAC if advertisements are increased.
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D is the trap answer. D is actually weakening the argument(though not a strong weakener like c)

The Wheat Farmers Alliance, a political action committee, attracts 70 percent of its contributors from an advertisement requesting contributions placed only in the September, October, and November issues of the Grange Report, a monthly newsletter for wheat farmers. The president of the Wheat Farmers Alliance, to increase the number of contributors, decides to advertise in each of the monthly issues of the Grange Report. She expects that, as a result of the additional Grange Report advertisements, the number of contributors will be increased to at least double the present number.

Which of the following, if true, would most strongly support the president's expectation?

(A) The September, October, and November advertisements were noticed by fewer than one-third of those readers of the Grange Report who would be willing to contribute to the Wheat Farmers Alliance. - CORRECT. The reach of advertisements increase with increase in the advertising months, eventually leading to higher probability of getting more contributors.
(B) Wheat farmers traditionally repay their bank loans in late summer after the winter wheat crop has been harvested and sold. - WRONG. Irrelevant at best OR more assumptions needed to be made for it be a good choice.
(C) The majority of the readers of the Grange Report with a great enough interest in the Wheat Farmers Alliance to contribute have already responded to the advertisements. - WRONG. Weakens instead.
(D) Most of those who contribute to the Wheat Farmers Alliance in the course of a year do so in response to advertisements in the Grange Report. - WRONG. Similar language but different result. It results in increase in contributions for the set when instead we seek the increase for its subset.
(E) The total number of readers of the Grange Report is stable from year to year. - WRONG. Yes, then what!! How it would lead to more contributions? Not clear enough from this choice.

Answer A.
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Please help me understand why 'A' & not 'D'?

Let's first consider (D).

(D) Most of those who contribute to the Wheat Farmers Alliance in the course of a year do so in response to advertisements in the Grange Report.

The easiest way to eliminate (D) is to notice that the passage says the following:

The Wheat Farmers Alliance, a political action committee, attracts 70 percent of its contributors from an advertisement requesting contributions

Since "70 percent" is "most of those who contribute," (D) simply repeats what the passage has already stated. We can't strengthen an argument by with information that has already been presented.

Now, let's consider (A).

(A) The September, October, and November advertisements were noticed by fewer than one-third of those readers of the Grange Report who would be willing to contribute to the Wheat Farmers Alliance.

This information confirms that there are enough additional readers who would contribute for the present number of contributors to be doubled. With the question of whether there enough additional readers answered in the affirmative, we have more confidence that the plan will work. In other words, by eliminating the possibility that there are not enough, we've increased the probability that the plan will work.
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