有一题不明白

antonia

有一题不明白：
  补充教材p168 34。
  A recent survey of all auto accident victims in Dole County found that, of the severely injured drivers 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 front-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?
  这道题应该是找Assumption，我蒙正确了，是A: Of all the drivers and front-seat passengers in the survey, more than 20% were wearing seat belts at the time of their accidents。但如何分析？

antonia

thanks

linsonstudy

首先要注意的是，句子中是 Of the severely injured ...（受重伤的），而A中是Of all the ... （所有的，包括受重伤的、受轻伤的和不受伤的）。
因为必须受轻伤的和不受伤人中系安全带的比例大于受重伤的人系安全带的比例，才能得出最终的结论。A的意思正好弥补了这一点。
这题是GMAT的特殊题，一般不会出这么难的，新东方的教师都说可以不用管它。