Remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it does strengthen, we negate the answer choice to see if the negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.
We know that twenty years ago the Republic of Rosinia produced nearly 100 million tons of potatoes, but last year the harvest barely reached 60 million tons. The conclusion in the argument is that agricultural researchers, who have failed to develop new higher-yielding strains of potatoes, are to blame for the decrease in potato harvest in Rosinia. Why? They have been concerned only with their own research and not with the needs of Rosinia. We are dealing with a straightforward cause and effect argument. The observed effect: decrease in potato yield. The cause: researchers not developing new higher-yield strains of potato since they were only concerned with their own research and not with the needs of Rosinia.
(A) is incorrect because it is irrelevant. We are blaming the agricultural researchers for the drop in potato yield last year. Any attempts they make currently will only affect next year's yield, not last year's. Thus, it still stands that they are to blame. The answer choice has not done anything to the argument.
(B) is CORRECT. First, we see that answer choice (B) strengthens. The argument points out that the researchers are to blame because they did not develop any NEW higher-yielding strains of potatoes. (B) strengthens this argument by pointing out that the strains of potatoes most commonly grown in Rosinia could not have produced the yields last year that they once did. If the previous potato strains were unable to bring about such yields, it would strengthen the idea that the researchers are to blame for the decrease in yield by not developing new higher-yielding strains of potatoes. Now, let's negate to make sure (B) is also necessary to our argument: Strains of potatoes most commonly grown in Rosinia could have produced the yields last year that they once did. If this were the case, then the researchers, who failed to develop new higher-yielding potato strains, would not be to blame. Thus, the argument would fall apart so (B) is our correct answer.
(C) is incorrect because it is irrelevant. It does not strengthen the argument because it is completely unrelated to the issue of decrease in potato yield last year due to researchers being only concerned with their own research as opposed to the needs of Rosinia.
(D) is incorrect because it weakens the argument. If wide fluctuations are not unusual, then the researchers are not completely to blame for the decrease in potato yield. It is possible that they did have Rosinia's needs in mind, and yet, since it is not unusual for wide fluctuations, there was nothing to be done. Notice that when (D) is negated, i.e. "wide fluctuations in size of potato crop over a twenty-year period are unusual," strengthens the argument. However, we are looking for an answer choice that strengthens but then destroys the argument when negated.
(E) is incorrect because it doesn't strengthen the argument. It is completely irrelevant. It does nothing to help prove that the agricultural researchers are to blame for the decrease in potato yield.
Hope that was helpful! Let us know if you have any other questions.
