阅读下面两篇短文，根据短文内容，选择最佳选项。
A 10-year-old boy decided to study judo (柔道), although he had lost his left arm in a car accident.
The boy began lessons with an old Japanese judo teacher, Mr. Tanaka. The boy was doing well, but
his teacher had taught him only one move in three months of training.
"Mr. Tanaka," the boy finally asked, "shouldn't I be learning more moves?"
"This is the only move you know, but this is the only move you'll ever need to know," his teacher
replied.
Several months later, Mr. Tanaka took the boy to his first tournament (锦标赛). The boy won his
first three matches, using his one move to win. The boy was now in the finals, and was amazed by his
own success.
This time, his opponent (对手) was bigger, stronger, and more experienced. The referee (裁判)
wanted to stop the match.
"No," the teacher insisted, "let him continue." Soon, his opponent lowered his guard. At that moment
the boy used his move to pin (钉住) him. The boy won the match and became the champion.
On the way home, the boy and his teacher reviewed every match. Then the boy asked what was
really in his mind.
"Mr. Tanaka, how did I win the tournament with only one move?"
"You won for two reasons," his teacher answered. "First, you've almost mastered one of the most
difficult throws in all of judo. And second, the only defense (防守) for that move is for your opponent to
catch your left arm."
The boy's biggest weakness had become his biggest strength (优点).
1. The boy decided to study judo though .
A. he was only ten years old
B. he would have an old teacher
C. he had lost his left arm in a car accident
D. he knew he was not clever
2. How did the boy feel when he was in the finals?
A. Proud.
B. Surprised.
C. Worried.
D. Happy.
3. What can you infer (推断) from the passage?
A. The boy wasn't doing well at first.
B. Mr. Tanaka is a good teacher because he knows his student's strength and weakness.
C. The referee believed the boy would win.
D. The boy had predicted that he would win.
的固定斜面上放置一质量M=1kg、长度L=3m的薄平板AB。平板的上表面光滑，其下端B与斜面底端C的距离为7m。在平板的上端A处放一质量
的小滑块，开始时使平板和滑块都静止，之后将它们无初速度释放。设平板与斜面、滑块与斜面间的动摩擦力因数均为
，求滑块与平板下端B到达斜面底端C的时间差
。（已知
） 
，故滑块在薄板上滑动时，薄板静止不动
，用时
