What Does HCPCS Level II Modifier AD Signify

What does HCPCS Level II modifier AD signify?

A) Physician providing a service in an unlisted health professional shortage area (HPSA)
B) Orally administered nutrition, not by feeding tube
C) Medical supervision by a physician; more than four concurrent anesthesia procedures
D) ESRD patient for whom less than 6 dialysis sessions have been provided in a month