图示结构当水平支杆产生单位位移时，B-B截面的弯矩值为( )。

A:2EI／l B:3EI／l C:EI／l D:不定值

计算题：有一铁块为圆锥形，底面半径为0.8m，高为1.5m，试确定它的形心坐标。

根据下表给定的工作间逻辑关系绘成的单目标双代号网络如下图所示，但其中的错误有（）。

A:节点编号有误 B:有循环回路 C:有多个起点节点 D:有多个终点节点 E:不符合给定的逻辑关系

电路如题3-28图所示，设触发器的初始状态为0，其在输入A和CP脉冲作用下输出Q端的波形为（）。

A:A B:B C:C D:D

两系统传递函数分别为。调节时间分别为t1和t2则（）。

A:t₁≥t₂ B:t₁＜t₂ C:t₁=t₂ D:t₁≤t₂

资料中失业率最高与最低的国家（地区），其幸福指数相差（）

A:22.1 B:27.4 C:28.5 D:31.4

Part B Directions: A. Study the following picture carefully and write an essay of 160--200 words. B. Your essay must be written clearly on the ANSWER SHEET 2. (20 points) C. Your essay should meet the requirements below:

[参考范文]
As is depicted in the cartoon entitled "a crying water tap", water is continually dripping onto the ground, regrettably coming to no use. On the floor is spread some water, which is absolutely wasted, and the scene points to a rather thought-provoking problem of resources waste in the present day society.
The drawer of the picture probably desires that the public should realize the severe shortage of fresh water and should get rid of their negligent attitudes towards its value. Nowadays, some people seem to be rather indifferent to fresh water shortage, not only because they believe that such resources come from inexhaustible supplies but also because they do not place a high value on valuable things at all. Even worse, they hold the same notions towards many other natural resources than water - electricity, paper, and food, just to name a few. Sad to say, those people never get round to noticing the sad scene depicted in the picture. It seems that they pretend not to belong to the global village or they have misconceptions about nature conservation.
Given the painful situation, it is high time that the importance of nature conservation be directed to the public not simply by means of guidance but also by way of law enforcement. Unless such an epidemic is checked, we will run into great troubles in store for us, and even worse, there will not be water dripping out of the tap any more.

设f(x)在[a，b]上具有二阶导数，且f"(x)≠0，f(a)=f(b)=0，试证： (Ⅰ) ∈(a，b)，有f(x)≠0； (Ⅱ) ∈(a，b)，使f’(ξ)=f(ξ)．

若[*]∈(a，b)，使f(x₀)=0,f(x)在区间[a，x₀]上满足罗尔定理条件，则至少存在一点ξ_A∈(a，x₀)，使f’(ξ_A)=0．同理，存在一点ξ_B∈(x₀，b)，使f’(ξ_B)=0．于是f’(x)在区间[ξ_A，ξ_B]上满足罗尔定理条件，因此，至少存在一点ξ∈(ξ_A，ξ_B)，使f"(ξ)=0．这与题设f"(x)≠0相矛盾，故[*]∈(a，b)，有f(x)≠0．
(Ⅱ)令辅助函数F(x)=e^-xf(x)，则
F(a)=e^-af(a)=0，F(b)=e^-Ff(b)=0,
且由题设知，F(x)在[a，b]上连续，在(a，b)内可导，故由罗尔定理，至少存在一点ξ∈(a，b)，使F’(ξ)=0，即
[*]
因e^-ξ≠0，故f’(ξ)=f(ξ)=0，即
f’(ξ)=f(ξ)．[分析] 欲证(Ⅰ)，正面证明不易处理，可采用反证法，结合罗尔定理加以证明．证明(Ⅱ)，先构造辅助函数，再用罗尔定理即可．

这个标志是何含义？（）

A:右侧通行 B:左侧通行 C:向右行驶 D:环岛行驶

如图标志的含义是什么（）

A:禁止车辆临时停放 B:停车让行 C:禁止车辆临时或长时停放 D:禁止机动车驶入