[求助]这道题选什么

题目是从费费的网络课堂资料里找到的：

14. 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?
(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.
(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.

给的答案是A。但是我非常得不理解。从题目来看，80%的重伤员是没有带安全带的，这就可以直接推出没有带安全带在车祸中很可能受重伤。也就是说开车时带安全带可以减少重伤的可能性。not...unless是一个A推B的题目，从这个结论中必然推出什么呢？我看不出A的正确在哪里，请各位答疑

大全-2-14

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