The research is so designed that once _______ nothing can be

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The research is so designed that once _______ nothing can be done to change it.
A.begins B.having begun
C.beginning D.begun

D 等于 The research is so designed that once it is begun nothing can be done to change it.
once表示一旦怎么样就.once在这里用法等于when,作为从句的引导词,所以作为修饰性从句的主语谓语（it is)可以省略.
你也许以后会遇到这样的考法:Once ____,the research can not be changed by anything.也是填begun.道理是完全一样的.

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