A professor conducts a study on tipping behavior in restaurants and hotels, which is designed to determine whether factors such as posture and communication influence tips left by customers. He conducts this study based on his experience in working at a restaurant as a youth. From this scenario, it can be inferred that the initial idea for the professor's study stems from: A. past research.B. his common sense and intuition or gut feeling.C. a scientific theory.D.his observation of an actual event.