Psychologists have studied the impact a person’s attitude has on his ability to accomplish tasks. In one study, a group of college students was outfitted with contraptions designed to administer soothing heat pulses to the students’ neck and shoulder muscles. The students were told that the pulses would enhance their performance of in-class assignments. Only half of these contraptions worked, but neither the students nor the psychologists were told which students would actually receive these pulses. This component of the experiment is often frustrated, however, because
Which one of the following selections, if it is true, completes the sentence most appropriately?