Many women suffer from anemia. A female physician who is also an avid jogger wanted to know if women who exercise regularly have a different mean red blood cell count than women who do not. She also wanted to know if the amount of a particular iron supplement a woman takes has any effect and whether the effect is the same for both groups. Write a model that will reflect the relationship between red blood cell count and the two independent variables just described, assuming that

a. The effect of the iron supplement on mean blood cell count is the same regardless of whether a woman exercises regularly.

b. The effect of the iron supplement on mean blood cell count depends on whether a woman exercises regularly.