Something tricky looking forward to discussion:

isuffering

Blood banks will shortly start to screen all donors to 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 the blood at the frequency for all donors.
Answer is A

Some one said that choice C can be ruled out just by the irrelative information, the word “underestimate”. But it’s not convincible to me, and I think there’s some trick in Choice C under the information given by the passage.

The trick in the passage I can’t be sure is as follows,
Is “all prospective blood donors” = “actual donors” here?
Or (*) “all prospective blood donors”= “actual donors” plus “the number of donors who would be disqualified by tests for NANB hepatitis”? My understanding is the latter (* )(?).

Then,
Since in the passage, “two-thirds of donors carrying NANB hepatitis” =
         “10 percent of actual donors”

Then  “one-thirds of donors carrying NANB hepatitis”= “5 percent of actual donors”

And since in the passage, “two-thirds of donors carrying NANB hepatitis” would be
       missed i.e., un-disqualified by tests for NANB hepatitis

then  “the number of donors who would be disqualified by tests for NANB hepatitis”=
        “one-thirds of donors carrying NANB hepatitis”=
        “5 percent of actual donors”

Again since in the passage, “the estimate of the number of donors who would be
        disqualified by tests for   NANB   hepatitis” = “ 5 percent of all prospective   
        blood donors”

And since (*) through the passage, “all prospective blood donors”=
      “actual donors” plus “the number of donors who would be disqualified by tests  
      for NANB hepatitis”(my understanding)(* )(?)

Then  “all prospective blood donors”> “actual donors”  

Then “ 5 percent of all prospective blood donors”>“5 percent of actual donors”

Then “the estimate of the number of donors who would be disqualified by tests for
        NANB hepatitis” should be an overestimate, not an underestimate

That is,
“the estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an overestimate*” is an assumption.

While
“the estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate*” is not an assumption.

Thus Choice C is not the right answer.

So my question is that
Is “all prospective blood donors” = “actual donors” here?
Or “all prospective blood donors”= “actual donors” plus “the number of donors who would be disqualified by tests for NANB hepatitis”?

Looking forward to discussion, thanks.

licheng

this is typical GMAT question..
just remember the structure.

StephanieYun

Actually , you have made a deep view into the sentences of the argument . The denation and connation of the argument have both been analysed clearly by your logic destruction and second-construction. I woudl highly respect your hard logic ability.

Based on my experience of the real Gmats, the CR would not be so difficult as to destructe its structure, as the way the above CR question is written.

Clearly , it would be easy to catch that the " alll perspective donars" is not equal to the " all actual donars" , in fact , "the number of  all perspective donars" is more than " the number of all actual donars" , as is inferred without confusion from the argument.

Strongly holding all these clues , we could make a inference and hypothesise about the assumption the argument has made .

isuffering

I see, so thanks .