Residents of three villages with three different sources of water supply were asked to participate in a survey to identify cholera carriers. Becuase several cholera deaths had occurred lately, nearly everyone agreed to be examined. The proportion of residents in each village who were carriers was computed and compared. What kind of study was this?a. Cross-sectional study
b. Case-control study
c. Prospective cohort study
d. Retrospective cohort study
e. Experimental study