题目内容：
回答题：Pain
All of ushave 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 cantake complete control of our body and mind, making it impossible to move and evento think．Yet we need pain．Without it, we would not know．If we have hurtourselves．It is our body' s warning system．(47)
Pain isthe 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 drags that stoppedsome pains．(48) But new knowledge about the process of pain ishelping them to control pain better.
Scientistshave learned that the sense of pain is made up of both chemical and electrical signals．(49) Scientists also havelearned that the nervous system sends two different kinds of pain messages tothe brain: one very fast, the other slow．The first message is the warningsignal．It moves at a speed of 30 meters a second In less than a second, thebrain understands that part of the body is hurt and how badly it is injured (50) It tells us not to use theinjured part until it heals.
A．Andothers have painful attacks all the time.
B．Thesesignals travel from nerve cells in the injured area, up the spinal cord (脊髓) to thebrain, and back down again.
C．It tells us that we are injured and should do something about it.
D．Theyknew little about the process of pain itself.
E．Theother message moves at a speed of only 0．13 meter a seconD．
F．And they send the second, slower message of pain to the brain.
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