为什么蓄冷器缩短切换时间可使冷端温差减小？

The earth is changing. Tomorrow it will be different. All the living things on the earth are changing. Even people are changing, and tomorrow you will be different.

There are many kinds of change, and many different things than cause change. People need to realize the changes, to learn to see them, and to learn what causes change.

Human beings must solve many problems so that they can continue to live successfully. War is one of these problems. Disease and natural disaster are problems. Having enough food and energy for the growing number of people in the world is a problem. Most problems can be solved, but some are impossible to solve. The way in which people solve problems affects how long and how well they can live.

As people solve their problems, they change things, and changes often cause new problems for people and for other living things on the earth. Sometimes the new problems seem even worse than the old ones. Human beings will probably never stop having problems to solve. Can you use your imagination to describe a world that has no problems at all ?

小题1:The reason why everything is changing is that______.

A．people want them to be changing

B．there’re many different things that cause them to change

C．the living things are changing

D．the world is too small小题2:People need to solve problems because______.

A．there will be more and more new problems if they don’t solve the old ones.

B．the problems are difficult to solve

C．they have to go on living

D．there are so many things that are changing小题3:New problems appear ______.

A．because old problems aren’t solved.

B．because people don’t solve them

C．because some people are foolish

D．when people are solving old ones小题4:Which of the following if wrong according to the passage ?

A．Disaster and war are two of the problems people have to solve.

B．People need to solve all the problems at the same time.

C．The way people solve problems has much to do with their life.

D．People can’t solve all the problem at one time.小题5:The underlined word “disaster” in the passage probably means “______” in Chinese.

A．灾难

B．资源

C．条件

D．环境

在进行导游活动时，无论遇到何种情况，导游人员均不得擅自中止导游活动。一般来说，构成中止导游活动必须具备的条件有( )。

A．必须在导游活动结束之前

B．必须是征得多数旅游者同意

C．必须是擅自中止

D．必须是彻底中止

E．必须是未向旅行社事先说明

飞行时间质谱仪可以根据带电粒子的飞行时间对气体分子进行分析。如图所示，在真空状态下，自脉冲阀P喷出微量气体，经激光照射产生不同正离子，自a板小孔进入a、b间的加速电场，从b板小孔射出，沿中线方向进入M、N板间的方形区域，然后到达紧靠在其右侧的探测器。已知极板a、b间的电压为U₀，间距为d，极板M、N的长度和间距均为L。不计离子重力及经过a板时的初速度。

（1）若M、N板间无电场和磁场，请推导出离子从a板到探测器的飞行时间t与比荷k（k=，q和m分别为离子的电荷量和质量）的关系式；

（2）若在M、N间只加上偏转电压U₁，请论证说明不同正离子的轨迹是否重合；

（3）若在M、N间只加上垂直于纸面的匀强磁场。已知进入a、b间的正离子有一价和二价的两种，质量均为m，元电荷为e。要使所有正离子均能通过方形区域从右侧飞出，求所加磁场的磁感应强度的最大值B_m。

流行病学实验中可比性较好的对照不包括（）

A.安慰剂对照

B.标准疗法对照

C.历史对照

D.自身对照

E.交叉对照

甲建筑公司的项目部对外承包工程，所签订的合同无效