下列各句中，划线成语使用恰当的一句是(       )

Dear friend,

    My name is Erick. I am 12. My birthday is on October 12 th. I am from America. I'm in Class 3, Grade 7. We are learning Chinese in our school. Our Chinese teacher's name is Wang Hao.

    Here are some of my friends. Jimmy is tall and he has black hair. He is good at playing basketball. He is a member of the Basketball Club. May is short and she wears glasses and a yellow dress. She likes English very much and speaks English very well. Sally is a black girl. She has short hair. She likes swimming. She swims best in the swimming team. I like music. I'm the boy with glasses.

Yours,

Erick

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密码子指的是（   ）