这可就怪了

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.（A）

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

答案先A，不解.

A is the best choice.

If A is not valid, donors carrying NANB have other diseases for which reliable screening tests performed. Then these people are possibily disqualified because they are carrying other diseases. As a result, the number of these people who are still able to supply blood are less than 10% of donors.