一个对D镇汽车事故幸存者调查的报告发现，在司机和前排乘客严重受伤的事故中80%的人在意外发生是没有带安全带。这表明，司机和前排乘客携带安全带能够很大程度上降低他们在汽车事故中受重伤的风险。

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

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

在这次调查中，超过20%的司机和前排乘客在事故发生时佩戴了安全带。

(B) Considerably more than 20 percent of drivers and front-seat passengers in Dole County always wear seat belts when traveling by car.

(C) More drivers and front-seat passengers in the survey than rear-seat passengers were very severely injured.

(D) More than half of the drivers and front-seat passengers in the survey were not wearing seat belts at the time of their accidents.

(E) Most of the auto accidents reported to police in Dole County do not involve any serious injury.

答案是A? 偶对A取非咋得不到结论不成立呢？

"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."这句话的意思也许就说：出车祸时佩带安全带受重伤的几率要小于出车祸时没有佩带安全带受重伤的几率。

也可以用一个不等式来表示，假设出车祸时佩带安全带的人数为A，出车祸时没有佩带安全带的人数为B，出车祸时佩带安全带受重伤的人数为C，出车祸时没有佩带安全带受重伤的人数为D。那么，C/A<D/B。所以也就是说要在下面选项中找到一个能够支持这个不等式成立的。

我们可以假设这次D镇调查的总人数为X，其中受重伤的人数为Y，或者为了方便判断，也可以直接把X设为100，把Y设为50，因为这个数值的多少对于本题没有影响。

根据A选项，前面不等式中C＝50×20％＝10，D＝50×80％＝40，A>100×20％即A>20，B<100×80％即B<80，所以C/A<1/2，而D/B>1/2，故该不等式成立。所以A选项是正确的。