Which of the following best describes the results of dr. sneed's study on volunteering and hypertension?

a. the risk of developing hypertension is positively correlated with number of hours spent volunteering.
b. the risk of hypertension is lower only for people who volunteer more than 200 hours per year.
c. volunteering has a minimal, if any, effect on rates of hypertension.
d. the risk of developing hypertension is negatively correlated with number of hours spent volunteering.