【填句补文】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.
