Residents of three villages with three different types of water supply were asked to participate in a survey to identify cholera carriers. Because several cholera deaths had occurred recently, virtually everyone present at the time underwent examination. The proportion of residents in each village who were carriers was computed and compared. What is the proper classification for this study?
a. Cross-sectional study
b. Case-control study
c. Prospective cohort study
d. Retrospective cohort study
e. Experimental study
Answer: a. Cross-sectional study