In a classic study in the investigation of the health effects of cigarette smoking, volunteers working in 1,121 counties in 25 states in the U. S. recruited 447,196 men, age 33-92 and had them complete a questionnaire about their smoking habit. Five years later, 39,178 of these men had died, and it was concluded that life expectancy was diminished by cigarette smoking. Which of the following best describes the design of this study? A. Case-control study
B. Prospective cohort study
C. Ecologic study
D. Cross-sectional study