The National Association of Fire Fighters says that 45 percent of homes now have smoke detectors, whereas only 30 percent of homes had them 10 years ago. This makes early detection of house fires no more likely, however, because over half of the domestic smoke detectors are either without batteries or else inoperative for some other reason. The conclusion is that smoke detectors are still not effective despite the increase in their prevalence because many of them don't function (due to lack of batteries or other issue).
In order for the conclusion above to be properly drawn, which one of the following assumptions would have to be made?
(A) Fifteen percent of domestic smoke detectors were installed less than 10 years ago. not relevant
(B) The number of fires per year in homes with smoke detectors has increased. not relevant
(C) Not all of the smoke detectors in homes are battery operated. There are also other reasons that they are inoperative.(D)
The proportion of domestic smoke detectors that are inoperative has increased in the past ten years. If just as many detectors were inoperative in the past, their current inefficacy can't be attributed to malfunction.
(E) Unlike automatic water sprinklers, a properly functioning smoke detector cannot by itself increase fire safety in a home. out of scope
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