"studies have shown that youth who listen to gangsta rap are often violent and show demeaning attitudes toward women. since gangsta rap is a bad influence on youth, the federal government should censor gangsta rap."

Martha participates in a research study in order to receive extra credit for her psychology class. when she arrives in the research office she is given a can of what looks and tastes like beer, but which actually contains no alcohol. in the adjoining room martha's classmate karen is also participating in the study. karen receives a can of alcoholic beer. if the experiment was designed to test the effects of alcohol on concentration, then who is exposed to the independent variable?