加强计量监督管理的核心内容是（）。 A.健全国家计量法制 B.解决国家计量单位的统一

如图所示，绝缘弹簧的下端固定在斜面底端，弹簧与斜面平行，带电小球Q（可视为质点）固定在光滑绝缘斜面上的M点，且在通过弹簧中心的直线ab上。现把与Q大小相同，电性相同的小球P，从直线ab上的N点由静止释放，在小球P与弹簧接触到速度变为零的过程中，以下说法正确的是  

A．小球P、小球Q、弹簧、还有地球组成系统的机械能不守恒  

B．小球P和弹簧的机械能守恒，且P速度最大时所受弹力与库仑力的合力最大  

C．小球P的动能、与地球间重力势能、与小球Q间电势能和弹簧弹性势能的总和不变  

D．小球P的速度先增大后减小

阅读理解。

     Mr. White is a teacher in our school. He is from Canada. He came to China two years ago. He is kind

and helpful. We all like him. His son, Rick, is my good friend. We are in the same class.

     Today is June 25th. It is Rick's fourteenth birthday. We plan to hold a party for him. Here is the program

for his birthday party. 

1. My classmate, Rick, comes from _________.

2. Rick's father is a _________.

3. Rick was born on _________ 25th.

4. Maria will perform _________ at the party.

5. _________ will play the piano because she is good at it.

为什么称单作用叶片泵为非卸荷式叶片泵，称双作用叶片泵为卸荷式叶片泵？

在关系代数运算中， (23) 运算结果的关系模式与原关系模式相同。

变压器带额定负荷时，BCH型差动继电器的不平衡电压不能超过（）mV。