螺杆泵的长度较大，在管理、检修与安装时应注意什么？

如图所示，两条平行的光滑金属导轨位于水平面内，距离为L=0.2 m，在导轨的一端接有阻值为R=0.5Ω的电阻，在x≥0处有一与水平面垂直的均匀磁场，磁感应强度B=0.5T，一质量为m=0.l kg的金属杆垂直放置在导轨上，以v₀=2 m/s的初速度进入磁场，在安培力和一垂直于杆的水平外力F的共同作用下做匀变速直线运动，加速度大小为a=2 m/s²，方向与速度方向相反。设导轨和金属杆的电阻都可以忽略，且接触良好。求：

（1）电流为零时金属杆所处的位置；

（2）电流为最大值的一半时施加在金属杆上的外力F的大小与方向；

（3）保持其他条件不变，而初速度v₀取不同值，求开始时F的方向与初速度v₀的关系。

Lane waited on the bench nearest the bridge, as he had been instructed.. The strong wind blew straight across the river, and he had to put his overcoat closer about him.

He went over his instructions in his mind. The spy would appear at four o’clock. He would talk to Lane for a while, after which he would get up, leaving his newspapers behind. The plans would be found inside.

A distant clock began to strike the hour. As if from nowhere, a man appeared and sat down beside Lane, placing his newspaper on the seat between them. He was thin and middle-aged, and seemed in need of a good meal. He didn’t look like a successful spy, Lane thought. His talk about the weather was uninteresting.

A few minutes later he got up and continued on his way. Lane picked up the paper which lay on the bench, as if he wanted to read the news. He was excited to see the plans, stuck on the center page. At that moment, however, there was a strong gust of wind which lifted the newspaper into the air, like a kite, and blew it into the river.

小题1:Which of the following statement is TRUE?

A．Sitting on the bench, Lane repeated his orders to himself

B．Lane would read about the plans in the newspaper.

C．The man Lane was expecting didn’t arrive on time

D．The man who suddenly appeared was not the one Lane was waiting for.小题2:The spy was going to give Lane some important        .

A．papers

B．information

C．instructions

D．advice小题3:Lane found the spy very         .

A．interesting

B．peculiar

C．ordinary

D．well-fed小题4: The spy left the newspaper        .

A．by the plan

B．by mistake

C．at Lane’s request

D．out of kindness

简述商业信用的作用与局限性。

《濒危野生动植物国际贸易公约》的英文缩写为（）。

A、CITES

B、WWF

C、IUCN

D、NASA

城市自来水供水企业和自建设施对外供水的企业，应当按照国家有关规定设置管网（），做好水压监测工作，确保供水管网的压力符合国家规定的标准。

A.检测点

B.监测点

C.测压点

D.观察点