A hypothetical study by Martin et al. 2018 examined between obesity and incident stroke. The unadjusted RR was 1.95. Stratification by gender revealed RRs of 1.45 and 1.46 for males and females respectively. In this study, gender is a
a. Confounder
b. Effect modification
c. Confounder and effect modifier
d. Information is insufficient to say whether confounder and effect modifier