Perhaps I am confused but the question you have picked out is a "flaw" question, not a "support" question. You are being asked to articulate a reasoning error that the argument has made. Answer choice C does not support the conclusion (but that doesn't really matter in this case) because it is simply irrelevant to the argument. It does not address the flaw in the argument; that is why it is wrong. Answer choice D is correct because it recognizes the possibility that computers and humans play chess differently. Therefore, it does not follow that a successful chess playing computer would have any effect on conceptions of human intelligence.
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Perhaps I am confused but the question you have picked out is a "flaw" question, not a "support" question. You are being asked to articulate a reasoning error that the argument has made. Answer choice C does not support the conclusion (but that doesn't really matter in this case) because it is simply irrelevant to the argument. It does not address the flaw in the argument; that is why it is wrong. Answer choice D is correct because it recognizes the possibility that computers and humans play chess differently. Therefore, it does not follow that a successful chess playing computer would have any effect on conceptions of human intelligence.