将甲、乙两个球随机放入编号为1，2，3的3个盒子中，每个盒子的放球数量不限，则在1，2号盒子中各有1个球的概率为 .
题目分析：甲、乙两个球随机放入编号为1，2，3的3个盒子中，每个球都有3种放法，故共有3×3=9种放法
在1，2号盒子中各有1个球，有2种放法
∴在1，2号盒子中各有1个球的概率为.
点评：本题考查排列知识，考查概率的计算，属于基础题．
| 阅读理解。 | |
| Do you like children? Do you have free time in the afternoon? We need a babysitter for our son. He's six years old. Hours are from 3:00 pm to 8:00 Pm, Monday to Friday. Sometimes you will work on weekends. The pay is $10 per hour. For the job, you will:
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| 1. What's the Chinese meaning for "babysitter"? | |
[ ] | |
| A.小孩 B.姐姐 C.幼儿 D.保姆 | |
| 2. The babysitter works for ______ hours from Monday to Friday. | |
[ ] | |
| A. 12 B. 5 C. 25 D. 15 | |
| 3. How much can the babysitter get if she works for three hours? | |
[ ] | |
| A. $10 B. $20 C. $30 D. $40 | |
| 4. The babysitter has to work in ______. | |
[ ] | |
| A. Shanghai B. Nanjing C. Beijing D. Dalian | |
| 5. Which of the following is TRUE? | |
[ ] | |
| A. The time for the job is from morning to evening. B. The babysitter has to watch TV with the kid. C. Sometimes the babysitter will work on weekends. D. The five-year-old boy needs a babysitter. |