The Focus on Neuroscience Box, "The Mysterious Placebo Effect," Described

The Focus on Neuroscience box, "The Mysterious Placebo Effect," described a PET-scan study on the placebo effect. One possible implication of the study's results is that placebos:

A) are not effective pain relievers.
B) should only be given in conjunction with the drug naloxone so as to avoid potential side effects.
C) reduce pain by activating the brain's natural opioid network.
D) increase pain by activating the brain's natural opioid network.