锅炉炉膛内有哪几种情况发生可燃混合物积存危险？

阅读短文，选择正确答案。

        Oftentimes one hears children wishing they were grown-ups, and old people wishing they were youngsters

again. Each age has its pleasures and its pains, and the happiest person is the one who enjoys what each age

gives him without wasting his time in useless things. 

        Childhood is a time when there are few responsibilities (责任) to think about. A child is usually fed, looked

after and loved by his parents. He will never be expected to do anything in return. In addition (另外), life is

always presenting (呈现) new things to a child, but the things that have lost their appeal (吸引力) for the old

because they are too well-known. In the meanwhile, a child has his pains: he is not so free to do what he wishes

to, he is continually (不断地) being told not to do a certain thing, or being punished for what he has done wrong.

        After a child grows up, he starts to make his own living. He can no longer expect others to pay for his

food, his clothes, or his room, and he has to work if he wants to live comfortably. If he spends most of his time

playing about in the way he used to play as a child, he will go hungry. And if he breaks the laws of society as he

used to break the laws of his parents, he may go in prison. If, however, he works hard, keeps out of trouble and

has good health, he can have the great happiness of building up his own position in society.

1. According to the second paragraph, the writer thinks that             . [ ]

A. life for a child is very easy

B. life for a child is very hard

C. only the old are interested in life

D. only children are interested in life

2. Which of the following is NOT TRUE according to the passage?[ ]

A. Parents hardly expect their children to do anything in return.

B. Children get interested in things around them.

C. Adults are free to do what they want to.

D. Adults should rely on themselves.

3. The main idea of the passage is that             . [ ]

A. life is never enjoyable since each age has some pains.

B. young men can have the greatest happiness.

C. childhood is the most enjoyable time in one's life

D. one can be a happy person only if he can enjoy each age of his life

4. The paragraph following this passage will most probably discuss             . [ ]

A. examples of successful young men

B. how to build up one's position in society

C. joys and pains of old people

D. what to do when one has problems in life

两个厚度分别为H₁和R₂而性质相同（如cv₁=cv₂）的土层，如果排水条件和应力分布条件也相同，则它们达到相同固结度时所需的时间t₁和t₂之比为（）。

A.A

B.B

C.C

D.D

天疱疮患者的口腔黏膜和皮肤表面常常形成（）

A.张力性大疱

B.松弛性大疱

C.大血疱

D.成簇小水疱

E.散在小水疱

一个空中飞行的物体具有80J的机械能，若它的重力势能为50J，则它的动能为          J；利用它的重力势能最多可做           J的功；若将它的重力势能全部转化为动能，则动能的大小为          J，此时它的机械能为            J．

含水分析仪是油在两个同轴电极之间流过时，电容量的变化同流体的绝缘常数变化成（）。

A.对应

B.倒数

C.比例

D.定值