What is the primary distinction between an observational and an experimental analytic epidemiology study? (a) researchers randomly allocate study participants to exposure or non-exposure group(b) its focus is to identify and quantify causal relationship between an exposure and an outcome(c) measures of association is rr or rd researchers decide the level of exposure among participants(d) researchers decide the level of exposure among participants(e) it require follow up of participants to ascertain the occurrence of disease of interest