Answer:A. differing densities of the elements that make up the Earth.
Explanation: The densities of the different materials making up the Earth surface and the Earth crust, due to the varying density of these materials it makes it difficult to break certain types of materials.
When the time required to break down materials differ,the earth's surface will have different layers caused by the time difference between the breakdown of these materials.
The more the density of the material the more difficult it is to break it down.