单选题

　　[]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.