Select the best definition of Coronary sinus.

A. The main venous channel that drains most of the myocardium into the right atrium. B. A network of spinal nerve roots C5 to T1 that supplies the upper limb. C. The motor nerve to the diaphragm, arising mainly from cervical roots C3 to C5. D. An arterial anastomotic ring at the base of the brain connecting anterior and posterior circulations.