求教大全2－19

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.

答案为A，但偶一开始就排除了A、B，从C、D、E中选C。看了答案还是没明白为何选A，求牛人指教，多谢。

A 项不看Do not，如果是削弱，则原A选项为题干的assumption

A项的否命题为：带有NANB的捐献者带有其他可以靠审查查出的疾病。

因此，可以得出，捐献者中没被查出NANB的人应该少于10%，削弱。

因此原命题为正确假设。