Na laboratory, smokers are asked to "drive" using a computerized driving simulator equipped with a stick shift and a gas pedal. the object is to maximize the distance covered by driving as fast as possible on a winding road while avoiding rear-end collisions. some of the participants smoke a real cigarette immediately before climbing into the driver's seat. others smoke a fake cigarette without nicotine. you are interested in comparing how many collisions the two groups have. in this study, the independent variable is