Answer: C) placebo
Explanation: In this experiment, the researchers used a placebo because neither the control group nor the experimental group knew if they were taking the real drug or the sugar pill without active ingredient.
A placebo is simply a medical or clinical procedure which is meant to trick participates in a clinical experiment that they are been given the trial drugs where as it was just a pills made from inactive ingredients, sometimes it has physical effects on the participants although these effects are more of a psychological phenomenon.