压力容器设计时为什么必须要考虑开孔的补强问题？压力容器接管补强结构主要有哪几种形式？

中央人民广播电台第二套节目的发射频率是720kHz，则该电波的波长多少米?

学校的中心工作是( )。

A．教学工作

B．行政工作

C．人文工作

D．后勤工作

United States President George W. Bush is expected to issue a directive in the next few weeks. It will give the US Air Force a green light for the development of space weapons, US media reported last week.

This would potentially start a new global arms race, some experts have warned.

To keep that from happening, last Wednesday the White House explained that it was not considering putting weapons in space. It said it was making a shift in US space policy to allow for protection of satellites.

But some defense analysts and arms control advocates argue that the policy will pave the way for the US to put both defensive and offensive  weapons in space.

“No one should be fooled,” said Theresa Hitchens, an American weapons expert.

The US is now restricted by a 1996 directive  signed by President Bill Clinton. Plans for space weapons were vetoed by the Clinton cabinet. The directive emphasized the peaceful use of space , in agreement with almost unanimous global opinion.

The US military has placed importance on space and has sent up numberous satellites for troop communications and to provide intelligence and data to guide bombs to their targets.

The US Air Force wants to develop space- based weapons that could strike targets anywhere in the world within 90 minutes of receiving the order to open fire.

These new weapons under development cover a wide range. They include hunter- killer satellites and orbiting weapons. And they use lasers, radio waves, or even dense metal tubes, known as “ Rods from God”, dropped from space to do the damage.

There are many barriers to the setting up of this kind of program, experts say.

First is the coast. It is estimated that the budget may be US $220 billion to US $ 1 trillion .

The technical difficulties of developing reliable space weapons are also a problem.

And, the program will draw strong criticism from around the world. Experts worry about starting a space arms race.

小题1:Which of the following might be the best title for the passage?

A．US Military Plans

B．The Global Arms Race

C．A Comparison between Two Presidents

D．President George W. Bush and the World Peace小题2:According to this passage, which of the following is NOT true?

A．President George W. Bush supports space weapon development.

B．The space weapon program will threaten world peace

C．Clinton government was poorer than Bush government.

D．The US space weapon project is very expensive.小题3:After reading this passage, it can be inferred that “___________ “.

A．George W. Bush’s directive to be issued will be against global opinions

B．The White House is right in explaining that US new space policy aims to protect satellites

C．US government has little difficulty in developing new space weapons

D．President George W. Bush and Bill Clinton share a lot in common小题4:The author’s attitude towards the coming space weapon development is _______.

A．optimistic

B．critical

C．excited

D．indifferent

如图甲所示，放在水平地面上的物体，受到方向不变的水平拉力F 的作用，F的大小与时间t的关系如图乙所示，物体运动速度v 与时间t 的关系如图丙所示，由图象可知，当t="1s" 时，物体受到的摩擦力为________N，当t="5s" 时，物体受到的合力为________N。

汽轮机低压缸为三层缸结构，请说出每层缸内叶片级数？叶轮呈何种结构？