[单项选择]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 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．1f 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 other slow．The first message is the warning signal．It moves at a speed of 30 meters a second．In les