To test the efficacy of vitamin c in preventing colds, army recruits are randomly assigned to one of two groups: one in which 500 mg of vitamin c is administered daily, and one in which 500 mg of a placebo is administered daily. both groups are followed to determine the number and severity of subsequent colds. what type of study design is this? cross-sectional study case-control study prospective cohort study historical prospective cohort study clinical trial community physical examination records of the incoming freshmen class of 1935 at the university of minnesota are examined in 1980 to see whether their recorded height and weight at the time of admission to the university are related to their chance of developing coronary heart disease by 1981. what type of study design is this? cross-sectional study case-control study prospective cohort study historical prospective cohort study clinical trial community trial