What is the study design that the investigators used? a. in 2002, investigators started a study of the association of cholesterol levels and stroke in a group of 2,000 healthy persons who had participated in a cholesterol screening program in 1992. b. the investigators determined exposure categories using cholesterol levels in all persons that were measured at the time of the screening program. c. a cutoff value of 200 mg/dl was used to define "high" cholesterol while those with levels below 200 were identified as having "low" cholesterol. d. using this definition, 1,000 persons had "high" cholesterol levels while the remaining 1,000 persons had "low" cholesterol. e. the investigators determined that 150 cases of stroke occurred by the end of 2004, with 113 cases occurring in the high cholesterol group.