阅读下列四篇短文，从每小题后所给的A,B,C或D四个选项中，选出最佳选项。
Have you ever noticed the colour of the water in a river or stream after a heavy rainfall?What do you think caused this change in colour?It is soil that has been washed into the river from the riverbank or from the nearby fields.
Components of Soil
Soil is made up of a number of layers(层),each having its own distinctive colour and texture. The upper layer is know as the litter. It acts like a blanket,limiting temperature changes and reducing water loss. The topsoil layer is made up of small particles of rock mixed with rotten plant and animal matter called humus(腐殖质),which is black and gives the topsoil its dark colour. This layer is usually rich in nutrients,oxygen,and water. Below the topsoil is the subsoil,a layer that contains more stones mixed with only small amounts of organic matter. This layer is lighter in colour because of the lack of humus. Beneath the soil lies a layer of bedrock.
Soil forms from the bottom up. Over time bedrock is attacked by rain,wind,frost,and snow. It is gradually broken down into smaller particles in a process called weathering. Plants begin to grow,and rotten materials enrich the topsoil. Most of the soil in Eastern Canada,for example,was formed from weathered rock that was exposed when the ice disappeared 12,000 years ago.
