Question

Which statement explains why fish spend a lot of energy removing oxygen from water?
A
They have to pump large volumes of water through their gills to keep their respiratory membranes moist.
B
The CO2 content of their tissues is much higher than that of terrestrial animals.
Their gills are covered with protective plates which make it more difficult for them to process the oxygen from
air.
D
They have to pump large volumes of water through their gills because of water's low oxygen content.​

Answer 1

Plus water is a lot heavier and thicker than air, so it takes a lot more work to move it around. The main reason why gills work for fish is the fact that fish are cold-blooded, which reduces their oxygen demands.