A study reports that regular consumption of brewed coffee is associated with a lower risk of heart disease, and the authors add, "These associations persisted after adjustment for cigarette smoking." What does this statement mean?


a. The investigators removed all smokers from the data, so the result applies only to non-smokers.
b. The investigators included cigarette smoking as a control variable, so the effect of coffee is estimated while holding smoking constant.
c. The investigators ignored smoking because it had no relation to either coffee consumption or heart disease.
d. The investigators proved smoking is not a cause of heart disease by adjusting for it.