Researchers have found that women who take oral contraceptives (birth control pills) are at higher risk of having a heart attack or stroke than women who do not take them. They also found that the risk is substantially higher for both groups if a woman smokes. Assume that the proportions of women who smoke are similar in both groups. In investigating the relationship between taking oral contraceptives (the explanatory variable) and having a heart attack or stroke (the response variable), explain whether smoking would fit the definition of each of the following:
a. A confounding variable.
b. An effect modifier.
c. An interacting variable.