logic question -HELP

Got this question from XDF logic book. Cannot understand the answer. Can anyone help me to explain how to find the right answer. Thanx.

A recent survey of all auto accident vicitims in Dole Couny found that, of the severely injured drivers and front-seat passengers, 80 percent were not weating seeat belts at teh time of their accidents. This indicated that, by eearing seat belts, drivers and front-seat passengers can greatly reduce their risk of being severely injured if they are in auto accident.

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

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

B) Considerly more than 20 percent of drivers and front-seat passengers in Dole County alwways wear seat belts when travelling by car.

C) More drivers and fornt-seat passengers in the survey than rea-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 teh time of their accidents.

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

A is correct!! Forget to put the answer..

Why D is not right?

这道题是数字计算，细心一点就好：

几个量要搞清楚—— 调查里的 司机及前排（A里面20%的基数）

                                受重伤的 司机及前排（题干里80%的基数）

                                所有的      司机及前排 （选项里迷惑人的）

D：50%*被调查的 和受重伤有联系吗？乘出来的东西没有用啊。得找一个选项，他的数字可以说明受重伤占〉50%比例

A：20%*事故里的（=题干里受重伤的）

20% * 80% = 16%?? not 60%.

搞不懂20%从那里推出来得。。  

噢！我晕了！是80%*（1-20%)>50%!就是题目里的80%和A里面的20% 啊