By pumping water over their gills, fish and aquatic mollusks?
a. keep their gill membranes moist
b. maintain their body temperature
c. keep themselves moving through the water
d. promote diffusion of oxygen and carbon dioxide

The correct answer is option D, Promote diffusion of oxygen and carbon dioxide

Explanation:

Animals which live in water get their oxygen from the water itself, however oxygen in water is at much lower concentration as compared to the air. Gill of these animals provide large surface area for the diffusion of oxygen across the gill surface. After absorption of oxygen, it is circulated through the circulatory system to the entire body. The diffusion through gills takes place due to the concentration difference between the oxygen level on the two sides of the gill membrane. There is low O2 concentration in the internal body as compared to the outer environment (i.e oxygen dissolved water ) and thus oxygen moves inward from region of high concentration to region of lower concentration.