How Did a Well-Known Social Psychologist,Robert Cialdini,learn About the Underlying

How did a well-known social psychologist,Robert Cialdini,learn about the underlying principles of compliance?

A) He did a study where he interviewed hundreds of salespersons.
B) He went undercover as a customer looking for a used car and noted how different salesmen attempted to sell various used cars to him.
C) He did a thorough,academic literature review of the field.
D) He went undercover and temporarily took jobs in settings where gaining compliance is critical.
E) He reflected on his previous profession as a car salesman for over a decade.