—Where are the papers for the Smith project?—They are in the folder ________ “S”．

—The police have caught the murderer.　　   —Yes, and he admitted ________ his boss on a rainy night seven years ago. 

 Upon ______ the office, he saw the message on his desk. 

Professor Smith told me that the book was well worth ________.

The problem of the widening gap between the rich and the poor, if not properly ______, can result in many serious social tensions.