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
网友回答
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。