What does the term 'Usual, customary, and reasonable' (UCR) refer to in the context of American healthcare?

A. A type of health insurance plan that covers 100% of medical expenses
B. A method of generating health care prices based on a fixed fee schedule
C. A government program that regulates healthcare costs
D. A method of generating health care prices described as 'more or less whatever doctors decided to charge'
Answer: 