To prevent the possibility that a placebo effect or researchers' expectations will influence a study's results,scientists employ

A) control groups.
B) experimental groups.
C) random assignment.
D) the double-blind procedure.

The double-blind procedure is designed to prevent both the placebo effect and the influence of the researchers' expectations on the outcome of the study. This procedure involves neither the participants nor the researchers knowing which participants are receiving the experimental treatment and which are receiving a placebo or alternative treatment. This helps to remove any biases or expectations that could affect the results. Control groups, experimental groups, and random assignment are all important aspects of experimental design but do not specifically address the issue of placebo effects or researcher expectations.