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Strengthen-CE. The correct answer choice is (E)
In this stimulus the author draws a causal conclusion based on anecdotal evidence. One researcher ingested a bacteria and later developed an ulcer. The author concludes that "it is highly likely that the bacteria strain induces ulcers." In order to strengthen this argument we must find the answer choice that either bolsters the causal claim directly, or rules out alternative explanations for the observed effect.
Answer choice (A): Since the author makes no claims whatsoever about kidney disease, this answer choice is completely irrelevant to the question of whether the bacteria induces ulcers.
Answer choice (B): The lack of other effects plays no role in the assessment of whether the researcher got the ulcer as a result of the bacteria, so this answer choice is incorrect.
Answer choice (C): If the bacterial strain has not been showed to induce ulcers in other animals, then it seems less likely that it does so in humans. Although this evidence is certainly no conclusive, it does weaken the author's argument to some extent.
Answer choice (D): The expertise of the researcher is irrelevant to the question of what caused that researcher's ulcer, so this answer choice is incorrect.
Answer choice (E): This is the correct answer choice. A large sample of people who did not have ulcers also did not have the referenced bacteria strain. This shows many examples where the cause is absent, and the effect is absent. This lends support to the causal argument in the stimulus.