Identify the type of study design (cohort, cross sectional, case control etc) f the following samples and justify your choices.

a. One third of University students were asked to list age, sex, smoking habits, weight and height. About 20% of the students provided complete information. About 10% of respondents had BMI over 25.

b. A total of 1,500 employees of a major aircraft company were initially examined in 1951 and were classified by diagnostic criteria for coronary artery disease (CAD). New cases of CDA have been identified by examination every three years and through death certificates. Attack rates in different subgroups have been computed.

c. A random sample of middle-aged sedentary adults were selected from four census tracts, and each person was examined for coronary artery disease (CAD). All persons without disease were randomly assigned to either a two years program of aerobic exercise or a two-year arthritis-prevention non-aerobic exercise program (Both groups were observed semi-annually for incidence of CAD).

d. Patients admitted for carcinoma of the stomach and patients without a diagnosis of cancer are interviewed about their chewing tobacco history to assess the possible association of chewing tobacco and gastric cancer.