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Flaw in the Reasoning. The correct answer choice is (B)
The stimulus concludes that since a switch from a high-additive to the low-additive diet is accompanied by a large drop in the percent of children displaying behavioral problems from an initial to a final observation, food additives must contribute to behavior problems in children.
The argument is unconvincing because there are far too many factors at play to credit, without further information, food additives as a cause of behavioral problems. Also, since there is no control group, you do not know whether it is possible, for instance, that most children would simply behave better the second time that they encounter a situation such as the study.
Since you are asked why the evidence fails to establish the conclusion, you should focus on the fact that the stimulus contains a causal argument that fails to give enough attention to highly plausible alternative causes.
Answer choice (A): It is true that since the high-additive diets could have differed from each other significantly, the proportional decrease in additives might differ widely. However, the stimulus merely proposes the very general conclusion that food additives contribute to problems, so whether the reduction is proportionate for each specific child is not critical.
Answer choice (B): This is the correct answer choice. Since the study does not include children who did not undergo a dietary change, it is not possible to rule out the alternative cause that children simply naturally behave better the second time in a situation, or behave better for other non-dietary reasons, so the conclusion of the stimulus does not follow.
Answer choice (C): The exact number of children in the study is irrelevant, and it is not a flaw to use proportions in a consistent fashion, so this choice is wrong.
Answer choice (D): If there is no evidence that the behavior of some children is unaffected by the additives, that is only more proof for the stimulus. The test-writers are attempting to catch testers who have difficulty with negations, and you need to read more carefully if you chose this response.
Answer choice (E): While it is true that the evidence is consistent with some of the children experiencing more frequent behavior problems on the low-additive diet, that is not a flaw. The stimulus only offers a general conclusion, so a few deviations do not matter. Furthermore, if the move from 60% behavior problems to 30% behavior problems included new problem-children in the 30%, that might suggest a greater benefit from lowering additive levels for the original 60% of children with behavioral problems, so it is unclear why this response should be taken to illustrate a flaw. This choice is wrong.