Dr Wisgoski Asked Fifty Drug Addicts a Series of Questions

Dr. Wisgoski asked fifty drug addicts a series of questions about their childhoods and then collected blood samples from them. He found a high correlation between levels of the stress hormone cortisol and amount of reported childhood trauma. Dr. Wisgoski concluded that childhood trauma causes high levels of stress during adulthood, which, in turn, causes drug addiction. Dr. Wisgoski's operational definition of adult stress level is

A) childhood trauma.
B) blood cortisol levels.
C) drug addiction.
D) self-report.