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.

【2017高考英语二轮复习限时训练31 阅读填空（1）】相关文章：

★ 2017届高考英语二轮复习课时训练：11（含解析）

★ 2017届高考英语二轮复习课时训练：16（含解析）

★ 2017届高考英语二轮复习热点限时训练：完形填空系列29

★ 2014届高考英语一轮复习课时作业(十一)必修2Module 5《Newspapers and Magazines》（外研版）

★ 贵州省贵阳市2014高考英语（新人教版）一轮阅读训练（34）附答案

★ 2017届高考英语二轮复习课时训练：17（含解析）

★ 2017届高考英语二轮复习课时训练：13（含解析）

★ 2017届高考英语二轮复习课时训练：3（含解析）

★ 2017届高考英语（江苏专用）二轮复习高考单词5大记忆法课件：4.联想记忆法

★ 2017届高考英语二轮复习课时训练：18（含解析）