The research is so designed that, once _______, nothing can be done to change it.
A．beginsB．having begunC．begunD．Beginning
D

試題分析：該題主要考查省略句的用法。譯文：這項(xiàng)研究就這樣定下了，一旦開始就無法改變。
句中once作“一旦……”講，引導(dǎo)一個條件句，出于這個條件句的主語與主句的主語（the research）一致，所以省略掉了主語it和系動詞is，只剩下一個過去分詞begun，故最佳答案應(yīng)選D。A項(xiàng)的錯誤在于缺少主語，B、D兩項(xiàng)都是現(xiàn)在分詞，不能用在條件句中作謂語，故可排除。
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