请教两题

14.   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 front-seat 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) Most of the auto accidents reported to police in Dole County do not involve any serious injury.

Blood banks will shortly start to screen all donors for NANB hepatitis. Although the new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors, they will still miss two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood.

19.   The argument above depends on which of the following assumptions?

(A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.

(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.

(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.

(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.

(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors.

答案是:A,A. 想不通，请指教,谢谢！

Q14: of all acidents surveyed, >20% wearing seat belts --> < 80% not wearing;  of severely injured, 80% not wearing.  So, not wearing increase the chance of survere injury.

Thanks, robertchu. I got 19

As for 14, I am still confused. Why is "<80% not wearing" needed, together with "80% of severely injured were not wearing", to come to the conclusion that "not wearing increases the chance of.."?  I think the fact that "80% of severely injured were not wearing" alone would contribute to the conclusion. I just could not think it through. Please help me.

q14.

I think author put the evidence in the other way. 20% wearing equals to 80% not wearing. He just reiterated the evidence. Is that right?

I think there is no reiteration - the "80%" in question is 80% of those severely injured. 20% (or 80%) in A is a portion of the whole sample.   

换个角度讲： In the whole sample, 大于20% wearing seat belts;  in the severely injured, 只有20% wearing 的受重伤．所以waring 降低了受重伤的可能性．
A并不是非常好，只是比其他选项好一点.