A racecar is equipped with a computer that records the reading on its speedometer every second during a race. If you graph this data, what will be the best choice for the dependent variable?

Since the device is a speedometer, the data it read is the speed of the racecar. Data recording involving time usually uses time as the independent variable. It was also said in the problem that it records the speed every second which shows that the time interval is constant. This means that only other data, the car's speed, is the dependent variable.