Which of the following BEST defines an "add-on code"?

A. Any of the numeric codes representing E/M services that may be reported only in conjunction with a different E/M service and may never be listed independently
B. Any category of E/M services that occur when the patient has been admitted to a hospital setting for the purpose of evaluation and/or treatment
C. The designation given when the patient's clinical presentation requires the physician performing the E/M service to closely monitor the patient's health before a final determination can be made
D. Any category of E/M services that occur in a clinical location that does not require the patient to be admitted for evaluation and management; most commonly identified as a clinical setting to which a patient presents for medical care and then leaves after the service is completed, such as a physician's clinic office

A