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