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(kāi)始就無(wú)法改變。
句中once作“一旦……”講，引導(dǎo)一個(gè)條件句，出于這個(gè)條件句的主語(yǔ)與主句的主語(yǔ)（the research）一致，所以省略掉了主語(yǔ)it和系動(dòng)詞is，只剩下一個(gè)過(guò)去分詞begun，故最佳答案應(yīng)選D。A項(xiàng)的錯(cuò)誤在于缺少主語(yǔ)，B、D兩項(xiàng)都是現(xiàn)在分詞，不能用在條件句中作謂語(yǔ)，故可排除。
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