The Focus on Neuroscience Box "The Mysterious Placebo Effect" Describes

The Focus on Neuroscience box "The Mysterious Placebo Effect" describes a positron emission tomographic scan study of the placebo effect. How did the two groups of participants in the study differ?

A) One group received the actual opioid painkiller, and the other group received an endorphin-blocking drug called naloxone.
B) One group was injected with an opiate painkiller, and the other group was injected with telomerase.
C) One group was injected with an opiate painkiller, and the other group was injected with saline solution (a placebo) .
D) One group was injected with saline solution (a placebo) , and the other group received no injection or other treatment.