Anders: The physical structure of the brain plays an important role in thinking. So researchers developing "thinking machines" -- computers that can make decisions based on both common sense and factual knowledge -- should closely model those machines on the structure of the brain. Yang: Important does not mean essential. After all, no flying machine closely modeled on birds has worked; workable aircraft are structurally very different from birds. So thinking machines closely modeled on the brain are also likely to fail. In developing a workable thinking machine, researchers would ttherefore increase their chances of success if they focus on the brain' s function and simply ignore its physical structure. The statement "thinking machines closely modeled on the brain are also likely to fail" serves which one of the following roles in Yang's argument? ['a particular example illustrating a general claim', 'the main conclusion of the argument', 'a subsidiary conclusion used in support of the main conclusion', 'a principle of research invoked in support of the conclusion']