_____ that air conditioners cause so much pollution, I would never have bought one.

1. A.
   Had I known
2. B.
   I had known
3. C.
   Were I to know
4. D.
   Did I know

_____ 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

_____ that air conditioners cause so much pollution, I would never have bought one.

_____ that air conditioners cause so much pollution, I would never have bought one.

______ that the air conditioners cause so much pollution, I would never have bought one.

A. If I knew            B. Did I know           C. If I have known      D. Had I known