A study begun in 1984 with a group of 2,000 young persons, aged 20-24 years, who were asked about their smoking status. The occurrence of cases of chronic lower respiratory disease (CLRD) between 1999 and 2009 was studied in this group. This is an example of... (choose one best answer) O a prospective cohort study. a cross-sectional study. O a retrospective cohort study. a case-control study. O a clinical trial