—What do you think of your boss?—He ’s kind-hearted. Not only _______ what to do, but also he lent me money.

— When will you come to Daqing?— I will call you _____I arrive.

David didn’t give up ______ he had failed in the experiment four times. 

 Yesterday morning I was just leaving_______ the telephone rang.

Some physics problems in this book are _________difficult for the students _________work out.