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