Logical Reasoning [TSA-2016]

The health service urgently needs more blood donors to meet demands on its ‘blood bank’ for transfusions. Most of us ignore this need, but we are only too glad to receive the blood of other donors in emergencies. To boost the blood bank, transfusions should only be given to those who have indicated they are donors or agree to become donors. In this fairer system, receiving and giving within the blood service would become clear in everyone’s mind and the blood bank given all the donations it needs.
Which one of the following, if true, weakens the above argument?

A. Some people are ineligible to give blood.
B. There is greater demand for certain blood types.
C. A high proportion of existing blood donors have themselves had blood transfusions.
D. Blood donations are at an all-time low.
E. Many people are wary of receiving blood because of infections.

Kindly explain how C excludes?

The main argument of the passage is that to increase blood donors, only those
willing to give blood or those who have given blood should be eligible to receive
blood. They argue that this is a fairer system of blood donations. This system is
clearly not fair if there exist individuals unable to give blood so the answer is A.

The reason why option c is incorrect as this suggests many blood donors have had transfusions but does not say
anything about the number of blood transfusion patients who are donors.
Hope this helps!