When Mary Smith was a student, she always wanted to become a teacher because she liked children. When she was twenty-one years old, she began teaching in a primary school（小學(xué)）. She was a good teacher, and she got on well with the children in her class. They enjoyed her teaching.

　　One day one of the girls in her class asked her, “Miss Smith, why does a man's hair become gray much earlier than his mustache and beard(胡子) do?”

　　Mary smiled and said, “I'm sorry 1 don't know. Is it true that a man's hair becomes gray before his mustache and beard do?”

　　“Yes, Miss Smith,” answered Helen, “it has happened to my father.” Hearing this, the other children in the class laughed aloud.

　　Then one of the boys stood and said, “I know, men's hair becomes grey first because it's sixteen years older than their mustache and beard.”

[　　]

[　　]

A．Mary liked children very much when she was still a student

B．The children in Mary's class enjoyed her teaching.

C．The hair of Helen's father became gray before his mustache and beard did

D．The boy was very clever and his answer was right.