单选题：Pain　　All of us have felt pain. We have cut ourselves. We ha

Pain
　　All of us have felt pain. We have cut ourselves. We have been burned. Or we have had headaches. Some of us suffer pain rarely_______46)
　　Pain can take complete control of our body and mind, making it impossible to move and even to think. Yet we need pain. Without it, we would not know if we　have hurt ourselves. It is our body's warning system________­­­­_47)
　　Pain is the most common reason we go to a doctor. It is the most common reason　we take medicines. Until recently, however, most doctors knew of only a few drugs that stopped some pains_________(48) But new knowledge about the process of pain is helping them to control pain better.
　　Scientists have learned that the sense of pain is made up of both chemical and electrical signals_________(49) Scientists also have learned that the nervous system sends two different kinds of pain messages to the brain: one very fast, the ether slow.
　　The first message is the warning signal. It moves at a speed of 30 meters a second. In less than a second, the brain understands that part of the body is hurt and how badly it is injured________(50) It tells us not to use the injured part until it heals.
A．And others have painful attacks all the time.
B．These signals travel from nerve cells in the injured area, up the spinal cord (脊髓) to the brain, and back down again.
C．It tells us that we are injured and should do something about it.
D．They knew little about the process of pain itself.
E．The other message moves at a speed of only one meter a second.
F． And they send the second, slower message of pain to the brain.
请选择（46）处的最佳答案．