請依下文回答第 21 題至第 25 題The skin is an organ, just like the brain, the heart and the stomach, and since it’s an external organ, it’s the mostvisible one. The skin has important functions. First of all, skin helps protect us from extreme temperatures, damagingsunlight, and harmful chemicals. It also contains elements that help the body fight off infection. In addition, the skinmanufactures vitamin D which helps the body use sunlight to build healthy bones. The rich supply of nerves just underthe surface of the skin provides another important function---the ability to sense changes in temperature and feel whensomething touches the body. This sense of touch helps the brain know what is happening in the outside world. At thesame time, skin allows parts of the body to move freely.The skin is made up of three layers. The outer layer, called the ‘epidermis’, contains a tough material called‘keratin’ which is also found in the hair and nails. This strong covering is crucial in protecting the body. The epidermisis constantly growing as the mature outer part is rubbed off and new skin grows to replace it. As new skin takes fiveweeks to reach the surface, the outermost layer is actually all dead skin. This protective coating varies in thickness ondifferent parts of the body. For example, it is ten times thicker on the soles of the feet compared to the skin around theeyes since it needs to protect us from constant impact with the ground while walking.The layer of skin below the epidermis is called the ‘dermis’. The way it is constructed gives the skin its strengthand also allows it to stretch when necessary. In addition, the rich blood flow through this layer helps the body regulateits temperature by increasing blood flow to the skin to allow heat to escape when it’s hot outside, or by decreasingblood flow to raise the body’s temperature when it’s cold.Under the dermis is another layer called the ‘subcutis’. It includes a sheet of fat which serves as a reserve supplyof energy in case the body experiences a food shortage. The fat also protects the body from sudden changes in heat andcold, and defends the body against injury from knocks and falls. The skin is a truly amazing organ, one which bothconnects us with and protects us from the outside world.
21 What is the purpose of the article?
(A) To describe how the epidermis functions.
(B) To explain the importance of keeping the skin healthy.
(C) To describe the functions of the various layers of the skin.
(D) To show how people use the skin to express themselves to the world.