大全-2-14

A recent survey of all auto accident victims in doile 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 severly 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 fornt-seat passengers in dole County always wear seat belts when traveling by car.
(C) More drivers and front-seat passengers than rear-seat passagers were very serverly injured.
(D) More than half of the drivers and front-seat passangers 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 serioius injury.

为什么答案是A呢,怎么也想不明白，这到题应如何入手呢?这种问 percentage的题有何规律吗?