求助~

WayneCH2012

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.
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，但不知道怎么理解~请牛牛们帮忙~

Gracespa

其实这个问题是建立在“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.“

A 说那些有NANB毛病的捐赠血的人，大多数情况下，不携带其他的病毒，那些病毒会有可信赖的测试一直在进行测试的。其实就是因为有测试，而且没有测出因为由于NANB而附带的病毒，所以证明携带NANB的人数和文中说的人数一样多。

WayneCH2012

就是因为有测试，而且没有测出因为由于NANB而附带的病毒，所以证明携带NANB的人数和文中说的人数一样多。

还是不太理解这句话~能不能在解释的通俗易懂点~万分感谢！！！

irenechl

这道题是个数字相关的题，不妨转化为数学来解：

根据原文，有占总人数5%的人，血液不合格。注意，不合格的原因，可能是携带了NANB病毒，也可能携带除了NANB之外的其他病毒。
我们先假设，这5%的不合格者，携带的只是NANB病毒，那么：
设NANB病毒携带者占总人数的比例为X，则X*（1-2/3）=5%，得出X=15%，即，总人数中的15%，携带了NANB病毒，而根据原文，这15%的病毒携带者，有2/3在检查时被漏掉了，所以，有15%*2/3=10%的人，仍然捐了携带NANB病毒的血液——正好是原文结论，而该假设，正好是选项A。