问题补充：
---You ought to have warned him of the danger!
--- ! But he wouldn’t listen!
A. So did I
B. So ought you
C. So I did
D. So I have
答案：
A. So did I
B. So ought you
(答案→)C. So I did
D. So I have
解析：句意：你本应该警告他有危险！我第五警告了，但是他不听。ought to have done 本该做某事而实际上没有做，so + 主语+ 助动词，表示对上句的肯定，根据语境警告的动作在过去所以用助动词did，故选C。
