Answer:
Option (a).
Explanation:
Counter current flow may be defined as the flow of water in the opposite direction with the flow of blood in the body. This countercurrent flow is generally occurs in fishes.
The one main benefit of the countercurrent flow is the absorption of maximum oxygen as the water flows inside the body. Approximately, three time more oxygen is extracted during the counter current flow. The concentration gradient is maintained up to the whole length of gills.
Thus, the correct answer is option (a).
Anything that takes up space
