The answer is A. One way, which is the opposite direction of blood flow.
Fish breathe using a viscera. The fish's gills are in the form of thin sheets and contain many filaments and function to filter oxygen. The gills are only used by cartilaginous fish and true bony fish as a respiratory organ. The gills of the fish are divided into two equal parts, each part of the fish is separated by lamellae.
Lamellae are organs that contain many blood vessels and are shaped like plates. Blood vessels in humans also contain oxygen, the more active the fish, the more blood vessels it will need. In these blood vessels there is an exchange of oxygen and carbon dioxide. Then the fish will have more and more lamellae on its body to help the fish breathe.
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