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Assumption—CE. The correct answer choice is (B)
The conclusion in the stimulus is that schizophrenia is caused by damage to the physical structure of the brain. This is concluded based on the discovery that people with genetically identical brains showed a physical difference when one of the two had schizophrenia. Since we are looking for an assumption, we need to find an answer that would eliminate something that could severely weaken this argument (in this case, the possibility that the cause and effect are reversed).
Answer choice (A): The argument is not about the overall size of the brain, but only about specific portions and their relationship to schizophrenia.
Answer choice (B): This is the correct answer choice. As mentioned above, this answer eliminates the possibility that the cause (smaller portion of brain) could actually be the result of the effect (schizophrenia).
Answer choice (C): The comparison in the stimulus is between identical twins, so assumptions about their brains relative to the brains of others is irrelevant.
Answer choice (D): Again, the stimulus is discussing the comparison between one twin with schizophrenia and the other without, so both twins having (or not) schizophrenia is not relevant to the argument.
Answer choice (E): Since we are only concerned about the relationship between the size of a specific portion of the brain and schizophrenia, the frequency with which identical twins get schizophrenia is not important to the argument.