Questions 16-20 are based on the following passage:
  Firefighters are often asked to speak to school and community groups about the iomportance of fire safety.Particularly fire prevent 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 devieces in their homes.
  Specifically,they tell them these things.A smoke detector should be palced on each floor of a homek While sleeping,people are in particular danger of an emergent fire,and there must be a detector outside each sleeping area.A good site for an wmergent around above from the ceiling.
  Because of the dead-air space that might be missed by hot air bouncing aroud above a fire,smoke detectors should be  installed erther on the ceiling at least four inches from the nearest 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,entances,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.

What is the focus of this passage（）?

A.The proper instalation of home smoke detectors.
B.How firefighters carry out their responsibilities.
C.The detection of dead-air space on walls and ceilings.
D.How smoke detectors prevent fires in homes.