What is meant by counter-current?

• A. When blood flows in the opposite direction in two limbs of the Henle s loop
• B. When the blood flows in the same direction in two limbs of the Henle s loop
• C. When the blood does not flow through any of the limbs of Henle s loop
• D. When some blood flows in Henle s loop and the other blood flows in the vasa recta