单项选择题

Questions 16-20 are based on the following passage:
  Firefighters are often asked to speak to school and community groups about the importance of fire safety.particularly fire prevention and detection.Because smoke detectors reduce the risk of dying in a fire by half,firefighters often provide audiences with information on how to install these protective devices in their homes.
  Specifically,they tell them these things.A smoke detecetor should be placed on each floor of a home Whilse sleeping,people are in particular danger of an emergent fire,and there must be a detector outside each sleeping area.A good site for a detector would be a hallway that runs between living spaces and bedrooms.
  Because of the dead-air space that might be missed by hot air bouncing around above a fire,smoke dectectors should be installed erther on the ceiling at least four inches from the nearest wall,or high on a wall,or high on a wall at least four,but no further than twelve,inches from the ceiling.
  Detectors should not be mounted near windows,entrance,or other places where drafts（过堂风）might direct the smoke away from the unit.Nor should they be placed in kitchens and garages,where cooking and gas fumes are likely to cause false alarms.
 

 

A smoke detector moust always be placed（）.

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