Blood is very important. when doctors understand how blood goes around inside the body, they try ways of giving blood to people who need it. They take blood from the healthy people and give it to the patient in need of blood. This course is （ 1 ） “blood transfusion (输血)”.
   But there are two problems. First, it does not always work. Sometimes people die when they （ 2 ） blood transfusion. Later, doctors find that we do not all have the same kind of blood. There are four groups --- O, A, B and AB. We all have blood of one of these groups. They also find that they can give any kind of blood to people of group AB. But they find that they （ 3 ） give A-group blood to A-group people and B-group blood to B-group people. If a person has O-group blood and the doctor could give his blood to anyone else safely.
   There is another problem. To give blood of the right kind, doctors have to find a person of the right blood group。Often they cannot find a person in time. At first they find they can keep the blood in bottles for fifteen to twenty days. They do this by （ 4 ） it very cold. They find how to keep it longer in the end.         One day, when you grow up, you may decide to give blood to a “blood bank”. In this way, you may stop （ 5 ） dying. Or perhaps one day you may become ill and need some blood with the same kind.