阅读理解

　　When Mary was a student she always wanted to be a teacher because she liked children．When she was twenty-one years old,she began teaching in a small school．She was a good teacher and laughed a lot with the children in her class．They liked her teaching．One day,one of the girls in her class said to her,“Mary,why does a man's hair become grey before the mustache?” Mary laughed and answered,“I don't know,Helen．Do you know why it becomes grey before the mustache?” “I don't know,either,” answered Helen．“But it happened to my father．”

　　Then one of the boys said,“I know,Helen．Men's hair becomes grey first because it is sixteen years older than their mustache．”

根据短文内容，填入所缺单词。

　　Mary wanted to 　　1　　 a teacher when she was very 　　2　　．She 　　3　　 a teacher at the 　　4　　 of twenty-one．She 　　5　　 well and the children 　　6　　 her very much．One day 　　7　　 asked her a hard question．She wanted to know why 　　8　　 hair became grey before their mustache．It was hard for Mary to answer．But a boy could．He thought a man's hair became grey first just 　　9　　 a man 　　10　　 have mustache when he was young enough．

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