[求助]逻辑题一

oddyuibe

A recent survey of all auto accident victims in D County found that, of the severely injured divers and front-seat passengers, 80 percent were not wearing seat belts at the time of their accidents. This indicates that, by wearing seat belts, drivers and fromt-seat passengers can greatly reduce their risk of being severely injured if they are in an auto accident.

The conclusion above is not properly drawn unless which of the following is true?

1) Of all the drivers and front-seat passengers in the survey, more than 20 percent were wearing set belts at the time of their accidents.

为什么选这一个呢？这是什么意思呢？
多谢指教

maymayfang

我是用反证法做的，你会发现如果小于20%会矛盾，不知是不是最简便的方法