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I don't like this question. Please endeavour with solid explaintation.
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GMAT Club Legend
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I'll say E. I'll explain if right.
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Paul
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Yeah Only E makes sense. I look forward to Paul's explanation.
Hi Paul,
It is sunday!! Shouldn't you be outside your home??? 
- Anand.
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Guys try again little hard this time.
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This is a tough one. I would go with A then. If the 5% screened ones were screened because they had AIDS instead of NANB(although they carried both diseases), then it means that had that 5% carried ONLY NANB, they would/could not have been detected. The test would be futile. In other words, it ensures that the 5% screened out were screened out because the NANB was detected. I think this should be it.
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Paul
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my take on this one is E - based on the assumption that - if NANB donors will donate at an average frequency as all the donors.
From Evidence E- say there are 100 prospective donors if 2/3rd of NANB are missed then 1/3rd of NANB will be 5. (Are you with me). so
1/3 * NANB = 5 therefore actual NANB caught are 15. and the remaing are donors = 85
Now the conclusion C says - 10% of actual donors will still supply NANB ie 10% of 95 then the remaing donors are 95 - 9.5 ~ 85
All this can happen if the average frequency of donors is constant and we should be able to link the evidence with the conclusion.
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GMAT Club Legend
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Let me play the devil's advocate 
What if the 1/3 who are caught also have AIDS, a disease which can be caught with "reliable testing procedures", as A says? Now, let's take 15 persons who ONLY have NANB. Can we ascertain that the test will now be able to screen out that 5%, or 1/3 of NANB infected donors? (A) certainly casts doubt on the test result and we have to ascertain that the 5% screened out in the first place had ONLY NANB and nothing else.
Upon thinking about it again, the frequency of donation really does not matter since we are more concerned about the numbers of donors. All the given statistics are about the number of donors notwithstanding how frequently they give blood. Even though a NANB donor comes twice to give blood, he will be counted as two persons and will be part of the statistics already. This happens because even though he shows up twice, each and every time, the test has to be performed again (who knows what might happen in between the two donations  )
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Paul
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Bhai what is the official answer?
When I first looked at this problem, my first choice was A, after reading other posts I got confused. I think A makes sence because in order to come up with exact statistics, presumed condition must exist all the time. In this case no other test should disqualify anybody who would be qualified as donars by the new test.
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Praveen
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only B makes sense (2/3 of the donors still carry NANB disease).
rgds,
batli
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A it the OA.
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I disagree with Paul's explanation for this.
The stem is talking about blood banks which will screen for NANB hepatitis. I believe all the prospective donors will be tested. There is nothing about tests on people with AIDS mentioned. Even though Paul's reasoning is correct I believe it is too much to assume given the scope of the argument.
The argument should start with a statement saying all kinds of screening test on the donors will be done instead of saying just NANB hepatitis screening is done.
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I am not assuming anything here Anandnk. I'm just trying to cast doubt into the author's conclusion.  AIDS was just an example, it could have been with just any other disease. Author merely says that screening tests will eliminate 5% of all donors. Can we know for sure that those 5% eliminated were not eliminated because they also had other diseases instead of having ONLY NANB? By knowing that those NANB infected had one and only disease, NANB, only then can we say that the tests were effective and hence, the conclusion. Since the author did not mention that it was NANB tests which were performed, it could have been a general test which could have detected other diseases instead of NANB. We HAVE to know then that those screened out were ONLY NANB infected. I think I'm repeating myself here 
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Paul
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Straightforward A
Again the same technique: the opposite of A weakens the argument
From the stem we can conclude that 2/3 of donors will supply NANB contaminated blood (10% is a distraction here). This claim is invalidated by the opposite of A: if donors infected with NANB often carry other infections, the tests for those infections will disqualify these donors and thus reduce the share of bad donors
E is not good. We are talking about THE NUMBER of donours, not the number of times they donate blood. Hence, FREQUENCY is totally irrelevant
I like your questions, Bhai. They look like ETS (except Erich Schnitzler of course :)
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