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 studyb. case-control studyc. prospective cohort studyd. retrospective cohort studye. experimental study