请教OG 2题

1: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 screenting tests are rountinely 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 understimate.

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.

用有关无关排出应该能得出A，但是A如何更好的解释呢？

2：Theprice the government for standard weapons puchased from military contractors is determined by a pricing method called"historical costing"historical costing allows contractors to protect their profits by adding a percentage increase,based on the current rate of inflation,to the previous year's contractual price.

which of the following statements,if true,is the best basis for a criticism of historical costing as an economically sound pricing method for military contracts?

a;the government might continue topay for past inefficient use of funds.

b;the rate of inflation has varied considerable over the past twenty years.

(why a not b?)

thx.

1. 由two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors推出1/3donors carrying NANB hepatitis相当于5%actual donors，考虑到 95%all prospective blood donors=actual donors也就是all prospective blood donors近似等于actual donors，这样5 percent of all prospective blood donors=5%actual donors=1/3donors carrying NANB hepatitis，就推出答案。答案给了修正值in a large percentage of cases。

2. 题目应该是个weaken. B说the rate of inflation 变化可观，实际是support文中adding a percentage increase,based on the current rate of inflation,to the previous year's contractual price这个pricing method 。

a, 是说原来有不合理的因素past inefficient use of funds存在，这样如果用了这个pricing method （adding），就会放大这个不合理的因素，所以是weaken。

感谢感谢jnlvo,第二题好像不理解很白痴的样子，你的解释我很明白了。但是第一题我还是不明白，a中提到的carry other infections for which reliable screenting tests are rountinely performed 如何联系呢?

你的推理让人好佩服,但是如果考试时我肯定是不能如此有推理能力的.会晕掉的.嘿嘿.

有没有更好理解的解释呢?

第1题考试时我也许能猜对，就如你所说用无关排出，不过肯定不会想得太多。我写的那些也都是马后炮。

根据我上面的推理得到了一个结论5 %all prospective blood donors=5%actual donors=1/3donors carrying 也就是说5 %all prospective blood donors这些人（几乎）全是因为被查到了带有NANB才被排除的，且人数上等于1/3donors carrying NANB。否则就可能是a取反后的结果。

把a取反Donors carrying NANB hepatitis do, in a large percentage of cases, carry other infections for which reliable screenting tests are rountinely performed。就是带有（大量）NANB捐献者也带有其它infections，这些infections平时是可被reliable screenting tests检测的。这样就有可能5 %all prospective blood donors的绝大多数人是因为其它原因被排出的，而非是因为带了NANB。这样也就不能往下推出我原来的结论。例如只有0.1%的all prospective blood donors被查出带NANB，文中可知=1/3donors carrying NANB hepatitis。另外由2/3donors carrying NANB hepatitis= about 10 percent of actual donors这个条件推出1/3donors carrying NANB hepatitis=5%actual donors, 这样就得到0.1%all prospective blood donors=5%actual donors=5%X95%all prospective blood donors, 就推得0.1=4.75，这明显错。

所以a是assumption。

抱歉，我的表达能力有限，只能这样罗哩吧嗦一通。你看看我想的对不对，再讨论。

这题我又想了想，可能我是想过了。

A取反：（大量）带有NANB捐献者也带有其它平时是可被reliable screenting tests检测出的infections。这样直接削弱了miss two-thirds of donors carrying NANB hepatitis，因为这些人尽管没被the new screening tests 检测出，但可能被其它方法查出，同样地被排除掉，后面的结论about 10 percent of actual donors will still supply NANB contaminated blood也就不成立。