Why do many people suffer chronic pain long after an injury has healed? A. The brain has learned to increase its pain perception. B. The skin exhausts its supply of histamine. C. They took morphine...

Why do many people suffer chronic pain long after an injury has healed?
A. The brain has learned to increase its pain perception.
B. The skin exhausts its supply of histamine.
C. They took morphine too soon after a surgical operation.
D. The blood flow to the injured area did not increase.