The reason why you beat the gill cover is

The respiratory organ of crucian carp is gill, which is composed of many gill filaments. Because it contains a lot of capillaries, it is bright red. Water enters through the mouth and then flows out through the back edge of the gill cover. When water flows through the gill filament, dissolved oxygen in the water enters the blood vessels of the gill filament, while carbon dioxide is discharged into the water from the gill filament, and the two exchange gas.

So choose: a