Fish are more efficient in getting oxygen from water than oysters because?

Question

a. fish gills are in an internal cavity
b. oxygen diffuses directly into the blood of fish
c. water flows in one direction over fish gills
d. gill slits permit fish to swallow a lot of water

An explanation and the answer would be greatly appreciated!

Answer 1

The answer is C. In most fish, water enters the mouth and goes out the gill slits. When the water flows one way, the gills are always in contact with oxygenated water, and the fish can draw huge volumes of water with this arrangement.

In oysters, they have to suck the water in, hold it for a few seconds to let the gas molecules diffuse, and then pump it back out. So, roughly 1/2 the time, oyster gills are in contact with de-oxygenated water, and they can only inhale a small volume of water each breathing cycle.

Another difference is that fish do not need to actively pump water over their gills when they are moving, while oysters need to constantly pump water in and out, which requires them to use up some of the oxygen to power their muscles used in pumping.