that air conditioners cause so much pollution, I would never have bought one.
A. Had I known
B. I had known
C. Were I to know
D. Did I know
网友回答
(答案→)A. Had I known
B. I had known
C. Were I to know
D. Did I know
解析：考查虚拟语气。根据句意可知考查与过去事实相反的虚拟语气，从句用had + done，Had I known =If I had know，故选A。