两名教师与两名学生排成一排照相，则恰有两名学生排在两名教师之间的概率为（ ）
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两名教师与两名学生共4人排成一排，有
种情况，A 44
要使恰有两名学生排在两名教师之间，可以把2名学生看成一个整体，
先排2名教师共有
种方法，再把2名学生做全排列A 22
种方法插到2名教师中间，A 22
由分步计数原理可得：共有
×A 22
种方法．A 22
故所求概率为：
=
×A 22 A 22 A 44
=4 24 1 6
故选D
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| When Mary Smith was a 1 , she wanted to be a teacher, 2 she liked children. When she was twenty, she began teaching in a small school. She was a good teacher, and she 3 a lot with the children in her class. They 4 her teaching. One day 5 of the girls in her class said to her, "Miss Smith, 6 does a man's hair become grey before his beard (胡子) does?" Mary laughed and answered, "I don't know. Helen, do you know?" "I 7 , either, Miss Smith," said Helen, "but it happened to my father." The 8 children in the class laughed when they heard this. Then a boy said, "I 9 , Miss Smith. A man's hair becomes grey 10 because it's sixteen years older than his beard." | ||||
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