单选题 ______ had enough time, they would have visited the Great Wall.
A. Had they B. Have they
C. Would they have D. If they

【正确答案】 A

【答案解析】[考点] 虚拟语气
[解析] 本题是省略if的条件句的虚拟语气。由主句谓语动词的形式，可以判断是表示与过去事实相反的虚拟语气的句子，要求选的是条件句。如果用if开头，从句就应用过去完成时的形式，D选项不符合，排除。虚拟条件从句中的were，had或should可以前置而省略if。故选A。