I ________ not helping them when they were in difficulties．

I ________ not helping them when they were in difficulties．

A.have regretted

B.had had regretted

C.doregret

D.does regret

答案及解析： C

解析：

这句句子的意思是“我真的后悔没有在他们困难的时候帮助他们。”如果句子需要谓语强调，我们往往用do的助动词结构加上动词原形。