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 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 percent of drivers and frontseat passengers in Dole County always wear seat belts when traveling 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 front-seat passengers in the survey were not wearing seat belts at the time of their accidents.

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

由两个物体组成的物体系统，共具有（）个独立的平衡方程。

A.3

B.4

C.5

D.6