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补全短文：
Scientists have learned that the sense of pain is made up of both chemical and electrical signals. ________. Scientists also have learned that the nervous system sends two different kinds-of pain messages to the brain:one very fast,the other slow.
A.It tells us not to use the injured part until it heals.
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.The first message is the warning signal.
D.The other message moves at a speed of only one meter a second.
正确答案：B
答案解析：第一句讲的是疼痛的感觉是由两种信号signals 组成的，所以紧接着就提到两种信号的传播路线。B项恰恰照应了首句意思。
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