it was,i should say ,very kind of you to come all the way to help them.but you didnt have to .分析

题目

it was,i should say ,very kind of you to come all the way to help them.but you didnt have to .分析
分析一下句子各成分
其实我要问的不是翻译的问题，是道虚拟语气的题，后面有4个选择，didnt have to B,wouldnt have done C,mustnt have done D,mightnt have done,真确答案是A,
希望有英语高手从虚拟语气的知识点来阐述为何选A，因为从书上的知识点来说我觉得wouldnt have to 更好，

答案