Residents of 4 small towns with 4 different types of water supply were asked to participate in a study to identify cholera carriers. Because several cholera deaths occurred recently, virtually everyone present at the time underwent examination. The proportion of residents in each small town who were carriers was computed and compared. What is the proper classification of this study design? a. case-control study
b. cohort study
c. case series
d. randomized controlled trial
e. cross-sectional study