交流接触器工作噪音过大的原因有哪些？

声音分为乐音和噪声，电子琴能模仿各种乐器发出的声音，在技术上要解决的关键是能模仿各种乐器发出声音的______，噪声是当今社会的一大公害，它是由发声体的______产生的，有关资料显示，噪声每经过一条100m宽的林带可降低20-25dB，从减弱噪声的途径看，这是在______中降低噪声的．

ALA合酶的辅酶是（）。

A.尼克酰胺腺嘌呤二核苷酸

B.尼克酰胺腺嘌呤二核苷酸磷酸

C.黄素单核苷酸

D.磷酸吡哆醛

E.黄素腺嘌呤二核苷酸

香花石由前20号元素中的6种组成，其化学式为X₃Y₂(ZWR₄)₃T₂，X、Y、Z为金属元素，Z的最外层电子数与次外层相等，X、Z位于同主族，Y、Z、R、T位于同周期，R最外层电子数为次外层的3倍，T无正价，X与R原子序数之和是W的2倍。下列说法错误的是(    )

A．原子半径：Y＞Z＞R＞T

B．气态氢化物的稳定性：W＜R＜T

C．最高价氧化物对应的水化物碱性：X＞Z

D．XR₂、WR₂两化合物含有的化学键的类型相同

"I’ve changed my mind. I wanted to have a telescope, but now I want my dad back." Lucien Lawence’s letter to Father Christmas written after his father had been knifed to death outside his school gate, must have touched every heart. Lucien went on to say that without his father he couldn’t see the stars in the sky. When those whom we love depart from us, we cannot see the stars for a while.

But Lucien, the stars are still there, and one day, when you are older and your tears have gone, you will see them again. And, in a strange way, I expect that you will find your father is there too, in your mind and in your heart. I find that my parents, long dead now, still figure in many of my dreams and that I think of them perhaps more than I ever did when they were alive. I still live to please them and I’m still surprised by their reactions. I remember that when I became a professor, I was so proud, or rather so pleased with myself, that I couldn’t wait to cable my parents. The reply was a long time in coming, but when it did, all mother said was, "I hope this means that now you will have more time for the children.” I haven’t forgotten. The values of my parents still live on.

It makes me pause and think about how I will live on in the hearts and minds of my children and of those for whom I care. Would I have been as ready as Philip Lawrence have been to face the aggressors (挑衅者),and to lay down my life for those in my care? How many people would want me back for Christmas? It’s a serious thought, one to give me pause.

I pray silently, sometimes, in the dead of night, that ancient cry of a poet "Deliver my soul from the sword, and my darling from the power of the dog." Yet I know the death comes to us all, and sometimes comes suddenly. We must therefore plan to live forever, but live as if we will die tomor­row. We live on, in the lives of those we loved, and therefore we ought to have a care for what they will remember and what they will treasure. If more parents knew this in their hearts to be true, there might be fewer knives on our streets today.

小题1:according to the whole text we can see that the first paragraph ________.

A．puts forward the subject of the text

B．shows the author’s pity on the kid

C．acts as an introduction to the discussion

D．makes a clear statement of the author’s views小题2:In the second paragraph the author mainly wants to explain to us ________.

A．how much he misses his parents now

B．why his parents often appear in his dreams

C．when Lucien will get over all his sadness

D．how proud he was when he succeeded in life小题3: What feeling did the author’s mother express in her reply?

A．Proud.

B．Happy.

C．Disappointed.

D．Worried.小题4: In the author’s opinion, the value of a person’s life is ________.

A．to leave behind a precious memory to the people related

B．to have a high sense of duty to the whole society

C．to care what others will remember and treasure

D．to share happiness and sadness with his family小题5: What does the writer mean by the sentence taken from an old poem?

A．Call on criminals and murderers to lay down their guns.

B．Advise parents stay with their children safely at home.

C．Spend every day meaningfully in memory of the death.

D．Try to keep violence and murder far away from society.

变压器的额定容量为2MVA，变压器阻抗电压百分数为5.5，基准容量取 100MVA，变压器的电抗标幺值为( )。

A．2.75．
B．0.36．
C．0.03．
D．0.27