The soil near the surface is usually darker than about one meter down, it is because the top soil

Answer: Option C

The soil near the surface is usually darker than about one meter down, it is because the top soil is drier than sub-soil.
Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and it has a small number of rocks that are smaller mixed with it.
Topsoil is drier as it is in direct contact with the air and sunlight which makes it drier and darker.