问题补充：
They must have been here the day before yesterday,＿＿＿＿?
A. mustn’t they
B. didn’t they
C. mustn’t have they
D. had they
答案：
A. mustn’t they
(答案→)B. didn’t they
C. mustn’t have they
D. had they
解析：must have done表示对过去发生情况的肯定推测，句中带有明确的过去时间状语，故附加疑问句用“didn’t they”。
