【单选题】

You have been badly injured in a car accident. It is necessary to give you a blood transfusion because you lost a great deal of blood in the accident. However, special care must be taken in selecting new blood for you. If the blood is too different from your own, the transfusion could kill you.

There are four basic types of blood;A,B,A
B、and O.A、simple test can indicate a person’s blood type.Everybody is born with one of these four types of blooD、Blood type, like hair color and height, is inherited from parents.
Because of substances contained in each type, the four groups must be transfused carefully.Basically,A、and
B、cannot be mixeD、A、and
B、cannot receiveAB, butA
B、may receiveA、or
B、O can give to any other group, hence it is often called the universal donor. For the opposite reason,A
B、sometimes called the universal recipient. However, because so many reactions can occur in transfusions, patients usually receive only’ salt or plasma until their blood can be matched as exactly as possible in the blood bank of a hospital. In this way, it is possible to avoid any bad reactions to the transfusion.
There is a relationship between your blood type and your nationality.AmongEuropeans and people ofEuropean ancestry, about 42 percent have typeA、while 45 percent have type O. The rarest is typeA
B、Other races have different percentages. For example, someAmerican Indian groups have nearly 100 percent type O.
The word "hence" in line 10 means______.
A、always
B、oftenC、thereforeD、seldom