答案是A
这是虚拟语气的句子.Had they had enough time=== if they had had enough time --------这是对已经过去的事情的虚拟/ 假设.
== They didn't visit the Great Wall because they DIDN'T have enough time.时态前后一致吗？？可以用have they 吗？1 have they ------不可以用，因为关注的是以前的事情。2 时态前后必须一致：had had 搭配would have visited