一道逻辑题(吸取教训,一道道问).

30.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 sear 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 sear belts at the time of their accidents.

大家能不能告诉我为什么选A呢?

My reasoning is as follows:

No seat belt à severe injury

Equivalent to No severe injury à seat belt.

but it does not lead to Seat belt à no severe injury.

However, if we have proof that the remaining 20%, free of the severe injury, that means

Seat belt à no severe injury

The absence of seat belt makes a difference here!