No one knows exactly ______ this important discovery was made.

 _____ impressed me was the boy’s ability to deal with this kind of difficult physics problem.

--Sir, I feel it complicated to apply to a university abroad.--Why not read our guide? This is ________ the relevant information is available. 

--- Look at those clouds!     --- Don’t worry. _______ it rains, we’ll still have a great time.   

___ impressed me most about the woman was that she looked after the old man for twenty years without any complaint.