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:此题也为省略句。连词once后省略了主语the search,从句“once begun”相当于“once the search is begun”。答案为D。