This CR is tough. I will go with A.
Lets analyze the argument
Lets assume there are a 100 prospective donors. 5% of them will be disqualified as they carry the NANB virus. The tests will miss Two thirds of the donors having the NANB (1/3 = 5, so there are 15 donors with NANB, meaning 10 with NANB won't be disqualified). Therefore 10 percent of donors with NANB virus won't be detected.
if this 10% carried another infection, they would have been detected in other tests. Therefore this argument assumes that "Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed."
Phew... Sorry for the long boring explanation. Couldn't help it