单项选择题

Questions 11-15 are based on the following pssage:
  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,firefighter often provide audiences with information on how to install these protective devices 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 particular danger of an emergent fire,and there must be a detector outside each sleeping area.A good site for a detector would he 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 detectors should be installed either in 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 window,entrances,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 fums are likely to cause false alarms.

Compared with homes without smoke detectors,homes with them give their owners a 50% better chance of （）.

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