It was along the Mississippi River ________ Mark Twain spent much of his childhood.
——There must be a dozen pens in this house but I can never find one when I need one.——Keep looking. ____ is sure to turn up.
Who can you turn to in time of danger, it not ____.
——Which of the two books do you want?—— I want ____. Please show me .
Some people would rather ride bikes as bike ridding has_____ of the trouble of taking buses.
The Parkers bought a new house, but____will need a lot of work before they can move in.