问题补充:
The research is so designed that once nothing can be done to change it.
A. begins
B. having begun
C. beginning
D. begun
答案:
A. begins
B. having begun
C. beginning
(答案→)D. begun
解析:
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。