请教各位一道逻辑题：
a recent survey of all auto accident victims in Dole County found that, of the severely injured drivers and front-seat passengers, 80% 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% of drivers and front-seat passengers in Doel Conunty always wear seat belts when travelling 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 fron-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 conuty donot involve any serious injury | 
