Problem: You are doing a cohort study on the exposure of participation in weight training exercises in a population of people over age 60 and whether this helps bone mass density to prevent osteoporosis. According to the literature, calcium supplementation can also impact bone mass density (either with or without weight training). Construct a DAG that describes the above relationship, and indicate which variable (if any) you would need to control for to get a valid estimate of the relationship between weight training and bone mass density.