请教一道涉及数学百分比逻辑题

A recent survey of all auto accident victims in Dole County found that, of the severely  injured drivers and fron -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 severly injured if they are in an auto accident.

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

这道题的答案是: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.

请问为什么?

if A is ture:

            belt            no belt

severe       20                80 

no severe    >20               <80

多谢拉！

看不懂2楼的解释，可不可以用文字说明下。。。比例的分母不同怎么比较啊？

由题意写公式:  b/(a+b)=80% …(1)

由题中结论: a/(a+c)<b/(b+d)…(2)

将(1)代入(2): a/(a+c)<4a/(4a+d)   所以4c>d

再由选项A推: (a+c)/(a+b+c+d)>20%    也可以得出4c>d的结论
            

所以选A

这道题要是出在数学里我还能比较原谅ETS