Mobombi Had Completed About a Quarter of the Distance in the Marathon

Mobombi had completed about a quarter of the distance in the marathon in which he was a participant. Suddenly, he stumbled and fell. Despite feeling a sharp pain initially, he got up and continued to run until he completed the race. Upon crossing the finish line, he fell down writhing in pain. When checked out, it was discovered that Mobombi had broken his leg. He was able to run the remainder of the marathon relatively pain free due to the release of

A) neuropeptides.
B) amino acids.
C) monoamines.
D) endorphins.