Bill drives faster than the speed limit only when his friends are in the car with him because they tell him he is cool. he doesn’t speed when his parents or girlfriend are in the car because they don’t say it’s cool. what is the s d in this example

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?