Question

The research is so designed that once _______ nothing can be done to change it.

A. begins　　　　　　　　　 B. having begun

C. beginning　　　　　　　　 D. begun

 

Answer 1

D

由于 the research 与 begin 是被动关系，故用过去分词 begun。Once begun 可视为 once it is begun 之省略。