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 devives in their homes.
  Specifically,they tell them these things.A smoke detector should be placed on each floor of a home.While sleeping,people are in particaular 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 licing spaces and bedrooms.
  Because of the dead-air space that might be missed by hot are bouncing around above a fire,smoke detectors should be installed either on the ceiling at least four inches from the nearest wall,or a wall at least four,but no further than twelve,inches from the ceiling.
  Dectectors should not be mounted near windows,entrances.or other places where drafts（过堂风）might driect the smoke away from the unit.Nor should they be place in kitchens and garages,where cooking and gas fumes are likely to cause falae alarms.
 

One responsibility of a firefighter is to（）.