According to the passage, pocket calculators ______.

A、will help students think fast

B、will make math too easy for them

C、will do harm to students’ thinking ability

D、will contribute to the spread of new technology

在市场经济中，当苹果的供给量大于需求量时，解决苹果供需矛盾最有效的办法是（　　）。

A、降低梨的价格

B、让苹果的价格自由下降

C、对苹果进行出口

D、做苹果衍生产品

General Blair had been in the army _____ for 35 years when he retired from the navy.

A、service

B、work

C、job

D、homework

Which of the following would be the best title for this passage?

A、Types of Doctors in the United States

B、Health Care in the United States and Britain

C、Treatment of Sick Children in the United States

D、Medical Insurance in the United States and Britain

If you read this book, you will have a better _____ of the causes of trouble.

A、understanding

B、learning

C、knowledge

D、explanation

如图所示，导体棒ab、cd放在光滑的水平导轨上，cd棒通过滑轮悬挂一个质量为m的物块，整个装置处于磁感应强度大小为B、方向竖直向下的匀强磁场中，ab在外力作用下以速度v1匀速向右运动时，cd棒由静止释放，设ab、cd的长度均为L，ab棒的电阻为r1，cd棒的电阻为r2，导轨足够长且电阻不计，求：　（1）cd棒开始运动的方向与ab棒匀速运动速度v1取值的关系；　（2）稳定状态时，cd棒匀速运动的速度；　（3）稳定状态时，回路的电功率P电和外力的功率P外．