Problem: A longitudinal epidemiological study of risk factors for lung cancer was conducted. 500 smokers who smoked a pack of cigarettes for more than 30 years developed lung cancer, while 1400 smokers did not develop lung cancer. In the same study, 50 cases of lung cancer occurred among people who never smoked. The remaining 29,950 who never smoked did not develop lung cancer. Which of the following(s) most closely describes the type of epidemiologic study design used in the above scenario?