阅读理解。
There was a man who had a little boy that he loved very much. Every day after work the man would come
home and play with the little boy. He would always spend all of his spare time playing with the little boy.
One day, while the man was at work, he realized that he had extra work to do for the evening, and that he
wouldn't be able to play with his little boy. But, he wanted to be able to give the boy something to keep him
busy. So, looking around his office, he saw a magazine with a large map of the world on the cover. He got an
idea. He removed the map, and then patiently tore it up into small pieces. Then he put all the pieces in his coat
pocket.
When he got home, the little boy came running to him and was ready to play. The man explained that he
had extra work to do and couldn' t play with him, but he led the little boy into the dining room. He took out all
the pieces of the map and spread them on the table. He explained that it was a map of the world, and that by
the time he put it back together, his extra work would be finished and they could both play. Surely this would
keep the child busy for hours, he thought.
About half an hour later the boy came to the man and said, "Okay, it's finished. Can we play now?" The
man was surprised, saying, "That' s impossible. Let's go and see." And sure enough, there was the picture of
the world; all put together, every piece in its place.
The man said, "That's amazing! How did you do that?" the boy said, "it was simple. On the back of the
page was a picture of a man. When I put the man together, the whole world fell into place."
1. The man told his son to put a map back together because _____.
A. he had little time to play with his son
B. he was tired of playing with his son
C. he had to overwork in the office
D. he wanted his son to play alone
2. The son spent _____ putting the map back together.
A thirty minutes
B. several hours
C. the whole evening
D. the whole day
3. What can you infer from this story?
A. Children often make adults mad.
B. Adults know little about children.
C. Sometimes children surprise adults.
D. Adults always care for their children.
L。在紧靠平行板右侧的正三角形区域内存在着垂直于纸面的匀强磁场，三角形底边BC与PQ在同一水平线上，顶点A与MN在同一水平线上，如图所示。一个质量为m、电荷量为+q的粒子沿两板间中心线以初速度v0水平射入，若在两板间加某一恒定电压，粒子离开电场后恰好垂直于AB边进入磁场，并垂直于AC边射出。不计粒子的重力，整个装置都处于真空中。求： 
