阅读理解。

     Doctor Clark was a very busy man. He always worked hard. He often forgot his family's birthday.

He loved his wife. He wanted to give her a present. On his way home from his hospital he bough some

beautiful flowers in a shop. When Doctor Clark got home, he gave his wife the flowers and said "Happy

birthday to you, dear! It's September26 today, I remembered your birthday this year." Mrs. Clark

laughed and said, "My birthday was the day before yesterday. But thank you all the same. Better late

than never!"

1. What did Clark buy when it was his wife's birthday?

A. Some flowers  

B. Nothing  

C. Some food  

D. Some drinks

2. Where did Clark work?

A. In a shop  

B. In a family  

C. In a hospital  

D. In a store

3. What day is Clark's wife's birthday?

A. The day before yesterday    

B. Today

C. September            

D. September24

4. Why did he forget his family's birthdays? Because ______.

A. he is too old      

B. he is a doctor  

C. he is not busy    

D .he works very hard

如图所示，电阻不计且足够长的U型金属框架放置在绝缘水平面上，框架与水平面间的动摩擦因数μ=0.2，框架的宽度l=0.4m、质量m₁=0.2kg。质量m₂=0.1kg、电阻R=0.4Ω的导体棒ab垂直放在框架上，整个装置处于竖直向上的匀强磁场中，磁感应强度大小B=0.5T。对棒施加图示的水平恒力F，棒从静止开始无摩擦地运动，当棒的运动速度达到某值时，框架开始运动。棒与框架接触良好，设框架与水平面间最大静摩擦力与滑动摩擦力相等，g取10m/s²。求：

（1）框架刚开始运动时棒的速度v；

（2）欲使框架运动，所施加水平恒力F的最小值；

（3）若施加于棒的水平恒力F为3N，棒从静止开始运动0.7m时框架开始运动，求此过程中回路中产生的热量Q。