Using the Glasgow Coma Scale, which number indicates that the patient is in a comatose state?

A. A GCS of 15 indicates full consciousness and alertness. B. A GCS of 12 indicates moderate impairment of consciousness. C. A GCS of 6 indicates severe impairment, but not necessarily comatose. D. A GCS of 8 or below is considered to indicate a comatose state.