Fishes gills are engorge in blood , as water is pumped over it as the fish swims, it pulls water thru it's mouth and pushes it over it's gills extracting oxygen and the used water is ejected thru the fish slits opening that cover and protect the gill. the blood is circulated thru the fish body.
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Carbon dioxide, ammonia and methane
<span>The correct option is A. Dark age are so called because compare to other eras, historian don't know much about this era. This period of time is considered lost to history, no important record about the era has survived, The cause of the dark ages was the collapse of the Roman Empire and the invasions and raiding that characterized it.</span><span />