One characteristic that most health maintenance organizations (HMOs) share is that, typically, an HMO

a.reimburses subscribing members for their covered medical expenses

b.adjusts the amount of the subscribing members monthly premium to reflect the number of times the subscriber uses the HMOs medical services

c.requires subscribing members to pay a small co-payment, in addition to premium payments, for specified medical services

d.shifts all or part of the financial risk to the insurer rather than to the health care provider