Many hospitals have blood banks. In these banks, blood of all types is stored. When blood is needed, it is taken from the bank. Healthy people then give blood to the bank to replace   the blood that is used. Blood can be kept in the fridge for about three weeks with a kind of chemical. When people lose much of their blood because of operations or accidents, they may need transfusions . Some people may need transfusions because of illness. Sometimes only a part of the blood is used for transfusions. Plasma , the liquid part of the blood , may be given alone. Transfusions of plasma are often given when people have been badly burned. When a person gets a transfusion, he gets blood of the same type as his. Or there may be a bad reaction . To make it safer, before a transfusion is given, the blood to be used must be tested with the blood of the person who is to receive it. This testing is called cross matching of the blood. Oswald Robertson, an American doctor, was the first one to give transfusions. He used blood that had been stored for some time to help wounded soldiers in 1918 in World War One. Blood banks usually have   _  .
only one type of blood one or two types of blood two or three types of blood all types of blood