Why Is the Countercurrent Gas Exchange That Occurs in Fish

Why is the countercurrent gas exchange that occurs in fish an effective system for extracting oxygen from water?

A) Because water flows in one direction and blood in the opposite direction, oxygen can be removed from the gills efficiently.
B) Oxygen is able to diffuse out of water over the entire surface of the gills, because the blood always contains a lower concentration of oxygen than the water.
C) The hemolymph in this system delivers oxygen to the body efficiently after it enters the gills.
D) Countercurrent exchange is an active process, so it is able to move more oxygen into the blood than can simple diffusion.