A study in the U.S.A. in 1977 showed that the chemical ASA in aspirin could prevent a stroke.
A. carrying out B. carried out C. working out D. worked out
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解析：考查分词做定语用法。本题中的a study与动词短语carry out构成被动关系，故使用过去分词做定语来修饰名词。句意：由美国在1977年进行的研究显示阿司匹林里的ASA可以预防中风。故B正确。