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Weaken-#%. The correct answer choice is (A)
In this stimulus we see a common logical error: in comparing two options, the author presents a single benefit and concludes that the option which includes that benefit is preferable. Here, the author discusses the fact that about 12 new cases of polio arise each year as a result of the use of OPV, whereas a switch to IPV would cut this figure in half. Based on this limited information, the author concludes that it is time to switch to the IPV.
This sounds reasonable, but there might be more to the picture. Since this is a Weaken question, the correct answer choice will almost certainly present either a new detriment associated with IPV, or a new benefit associated with OPV.
Answer choice (A): This is the correct answer choice. As predicted, this correct answer choice introduces a detriment associated with the author-recommended IPV. In recent years there have been no new cases of naturally occurring polio in North America—if the switch were made as the author recommends, this number would increase from zero to "at least a few." Since this choice presents a detriment associated with IPV, it weakens the author's conclusion that we should make the switch.
Answer choice (B): The fact that most of those affected had weakened immune systems plays no role in assessing which vaccine to use, so this answer choice does not weaken the author's argument.
Answer choice (C): The rarity of the disease compared with other hazards is irrelevant; the stimulus is focused on how best to minimize even the very low current numbers.
Answer choice (D): What people currently use in different locations does not play into the author's argument, which concerns the question of which vaccine will produce the best results.
Answer choice (E): If most vaccines carry this same risk, then it is likely that OPV carries the risk as well. This would not weaken the author's argument unless we knew that OPV carried no such risk. Since we are not provided with any information regarding the OPV seizure risk, this answer choice does not weaken the argument and is thus incorrect.