Select the best definition of Portal vein.

A. A vessel that carries nutrient-rich blood from the gastrointestinal tract to the liver. B. The main venous channel that drains most of the myocardium into the right atrium. C. An arterial anastomotic ring at the base of the brain connecting anterior and posterior circulations. D. A network of spinal nerve roots C5 to T1 that supplies the upper limb.