D. Because....
The bulk density of a planet will reflect the relative amounts of the materials the world is made of, regardless of what materials occur at the surface. This is vitally important because when planets and moons form they start to differentiate. This means gravity causes the heavy elements to sink to the center of the world, while light elements float to the surface. So, the materials at the surface of a world represent the top-most sc um (a layer of dirt or froth on the surface of a liquid.) and not the true bulk of the planet's composition. The bulk density is the average of everything, inside and out, and it is a true indicator of the planet ¼s real composition.
Hope this Helps :D