单项选择题

Questions 16-20 are based on the follwing passage:
  Firefighters are often asked to speak to school and community groups about the imprtance of fire satety,particcularly fire preventions and detection.Because smoke detectors reduce the risk of dying in a frie by half,firefighters often provide andiences with information on how to install these protective devices in their homes.
  Specifically,thet tell them these things.A smoke detector should be placed ong each floor of a home.Whilse sleeping.people are in particular danger of an emergent fire,and there must be a dectector outside each sleeping area.A good site for a detector would be a hallway that runs between livign spaces and badrooms.
  Because of the dead-air space that might be missed by hot bouncing around above a fire.smoke detecor should be installed either on the ceilng at least four inches from the nearest wall,or high on alwll on a wall at least four,but no further than twelve,inches from the ceiling.
  Detectors should not be mounted near windows,entrances,or other places where drafts（过堂风）might direct the smoke away from the unit.Nor should they be plaecd in kitchens  and garages,where cooking and gas funmes are likely to cause alarms.
 

 

The passage implies that dead-air space is most likely lcose to where a wall meets'a ceiling（）.

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