Earthworms need a source of oxygen to perform cellular respiration. The problem is that they lack lungs, so the oxygen they need is obtained by diffusion through their skin. Why do earthworms need to keep their skin moist?

A) The skin has a good blood supply, and water diffuses out of the blood.
B) To conserve water.
C) Oxygen must dissolve in water before it diffuses into the blood.
D) The moisture on the skin increases the surface area and allows sufficient diffusion.