[求助]大全-1-19和大全-1-20

Questions 19-20 are based on the following.
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.

20. Which of the following inferences about the consequences of instituting the new tests is best supported by the passage above?
(A) The incidence of new cases of NANB hepatitis is likely to go up by 10 percent.
(B) Donations made by patients specifically for their own use are likely to become less frequent.
(C) The demand for blood from blood banks is likely to fluctuate more strongly.
(D) The blood supplies available from blood banks are likely to go down.
(E) The number of prospective first-time donors is likely to go up by 5 percent.

不明白。为什么选A和D。怎么推理的？

19:

5%查出来了，漏了2/3, 那么5%就是1/3. 文章结论说10%漏了，这是个精密的数学推理，没有任何他因, 这个假设很简单就是排除他因. 是NOT+WEAKEN,你把A的NOT按上读一下文章，感觉是不是削弱. 如果有其他常规检查查出了病人的问题，是不是这个数学推理就不成立了。

20.我觉得20和19有点关系，因为照上面的结论, 文章说10%人漏了去献了血,19的结论很清楚了，其实是少与10%的人去献了，因此血库的血就少了。所以D说(可能)会减少.