The engineers must have worked out the design by the end of last week ？A. mustn’t theyB. h

问题补充：

The engineers must have worked out the design by the end of last week，？

A. mustn’t they

B. hadn’t they 

C. haven’t they

D. didn’t they

答案：

A. mustn’t they

(答案→)B. hadn’t they 

C. haven’t they

D. didn’t they

解析：时间状语by the end of last week表示的是过去的过去，所以该填过去完成时。