Maureen is doing a study on how people drive when no one else is in the car versus when there are passengers. How can Maureen observe drivers' behaviors?
A. By looking for differences of behavior when she is sitting in the backseat of the car versus when she is outside the car
B. By observing differences of behavior between people who are driving versus people who have a passenger in the car
C. By seeing differences of behavior when people drive by themselves to pick up someone versus how people drive after they picked up passengers
D. All of the above