Countercurrent exchange is evident in
A) the flow of water across the gills of a fish and that of blood within those gills.
B) the flow of blood in the dorsal vessel of an insect and that of air within its tracheae.
C) the flow of air within the primary bronchi of a human and that of blood within the pulmonary veins.
D) the flow of water across the skin of a frog and that of blood within the ventricle of its heart.
E) the flow of fluid out of the arterial end of a capillary and that of fluid back into the venous end of the same capillary.
Answer: A) the flow of water across the gills of a fish and that of blood within those gills.
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