Aresearcher wants to assess the effects of varying amounts of alcohol on ratings of perception of friendliness. three groups of people are given either one, two, or three beers to drink. three more groups of people are given either one, two, or three nonalcoholic beers to drink. then, all participants are shown a series of pictures of people's faces and are asked to rank the perceived friendliness of each face on a scale from 1 to 10. in this experiment, the independent variable is: a. the amount of alcohol consumed.