The crude association between occupational exposure and lung cancer (outcome) among the study sample was 8.4. when data were stratified by alcohol intake, the associations were 2.2 (for non-drinkers) and 14.5 (for drinkers). assume that all (crude, and stratified) measures of association were statistically significant. alcohol use was (use 15% difference rule):
a. an effect modifier
b. a potential confounder
c. a random error
d. a confounder