What is a portal system?

• A
  $A$ vein that starts from an organ and ends in the heart.
• B
  An artery that divides in an organ and reforms from the union of its capillaries.
• C
  Blood entering the heart after passing from the digestive tract to the kidneys.
• D
  $A$ vein that starts as capillaries in one organ and ends as capillaries in another organ before returning to the heart.