A research study finds a negative correlation between oral contraceptives and women's blood pressure. Which of the following confounding variables would explain the observed positive relation? a. High blood pressure is positively correlated with the use of oral contraceptives. b. Women who are more prone to high blood pressure tend to take oral contraceptives. c. Women who take oral contraceptives tend to be younger. Age is positively associated with blood pressure. d. High blood pressure causes people to age faster. e. More recent studies tend to show that oral contraceptives increase blood pressure