One Major Difference Between PPOs (Preferred Providers Organizations) and HMOs

One major difference between PPOs (preferred providers organizations) and HMOs (health maintenance organizations) is that

A) PPOs set rates for various medical services or procedures, while HMOs allow their doctors and clinics to set their own rates.
B) PPOs seek to reduce health care costs by controlling prices, while HMOs seek to reduce costs by restricting quantity consumed.
C) HMOs employ their own physicians or contract for specialized services with outside providers, while PPOs have arrangements with a network of providers.
D) HMOs seek to reduce costs by capping the rates for various services, while PPOs seek to ration health care by having waiting periods.