According to Focus on Neuroscience: the Mysterious Placebo Effect, One

According to Focus on Neuroscience: The Mysterious Placebo Effect, one possible explanation for the placebo effect is that it:

A) is ineffective for reducing pain.
B) can have a harmful effect on the central nervous system.
C) reduces pain by activating the brain's natural opioid network.
D) increases pain by activating the brain's natural opioid network.