Astudy examined the impact of thc (the active ingredient in marijuana) on various physiological and psychological variables. the study recruited a sample of 18 young adults who were habitual marijuana smokers. participants came to the lab 3 times, each time completing the entire series of tests and questionnaire, but each time smoking a different marijuana cigarette: one with 3.9% thc, one with 1.8% thc, and one with no thc (a placebo). the order of the conditions was randomized and participants were not told how much thc was in the cigarette they smoked until after all the data were collected. this is an example of