21. What evidence is there that emphasizing prevention may be the best approach to ensuring health and controlling medical costs? 
 a. Since the emergence of prevention-focused HMOs in the United States, the health of Americans has been improving and medical costs have declined.
 b. Prevention is always less expensive than treatment.
 c. Mandatory mammograms have reduced the per capita cost of health care in several states by more than 30%.
 d. Adults in England, where prevention is emphasized, are healthier than Americans on all health measures, and less money is spent on health care.