D

　　Hair is one of the commonest parts of the body and something that is given a lot of attention. You’ve had it since birth and you take care of it every day. But how much do you know about your hair?

　　The average head of hair is made up of 120,000 strands(根), each of which has a life expectancy of about three years. While some hairs are growing, others are resting, and others are falling out.

　　A hair is made up of two parts: the shaft(发丝)and the root. The shaft is a visible(or: seen) part. It has a complex structure. The root is the living part of the hair. It is here that cells develop and form the shaft. It also has a very complex structure.

　　Every day a normal head of about 120,000 hairs will lose between 50 and 100 hairs—or 30,000 hairs per year. Hair dies, falls out, and grows again. The life cycle is made up of three stages. First, a long and active growth period that lasts about three years—the hair grows. Second, a very short transition(过渡)period that lasts about three weeks, during which time the hair is inactive—the hair follicle shrinks. Third, a resting period of about three months—at the end of this period the hair is pushed out and lost. This cycle runs constantly throughout life.

　　A hair grows from 1 to 1.5 cm each month and its usual life expectancy is about three years. So very long hair is rarely seen. However, there are some people who do grow long hair.

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