为什么变压器大修时，要检查绕组压钉的紧固情况？

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变压器长期运行后，由于机械力的作用和绝缘物水分的扩散，绕组绝缘尺寸收缩，会使变压器的轴向紧固松动，从而降低变压器绕组的机械强度和稳定性，承受不住变压器二次侧短路时的电动力。因此，在大修时必需检查压钉的压紧情况。

阅读理解

阅读理解。
Any introduction to Peking Opera would not be complete without telling of Mei Lanfang. Traditionally only men performed in Peking Opera, including the female roles. And Mei Lanfang was the very best master of the miss. During his stage life, he combined the traditions of the past with his own creations, shaping a style of his own and giving birth to “The Mei Lanfang School”. He was also the first artist to introduce Peking Opera to an overseas audience, making it popular to the world. For half a century, Mei Lanfang was a household name in China. Mei Lanfang began his stage life at the age of 8. His teacher said he showed little hope because of his boring eyes. To improve this, he exercised them day after day. He would fly kites and look at them moving slowly in a blue sky. He also kept pigeons in order to look at them soaring higher and higher until they disappeared into the clouds. Thanks to his efforts, he managed to transform his dull eyes into a pair of bright and expressive eyes and win national fame before the age of 20, In over 50 years on the stage, Mei Lanfang played no less than 100 differents characters in the traditional Peking Opera performance. He completely changed both stage make-up and costumes, and enriched characters’ gestures. expressions and poses. He also wrote many new plays, designing the dances himself. The many dances he created form part of the great legacy that he left to Peking Opera. In 1930.Mei Lanfang started on a successful US tour. There his brilliant performances fascinated the audience, making them realize that Peking Opera was a theatrical forrn of great literary and artistic value.
1. Mei Lanfang was the first artist to introduce Peking Opera to_______.
A. China B. Beijing C. the world D. schools
2. When did Mei Lanfang begin his stage life?
A. At the age of 8. B. At the age of 20. C. In 1930. D. In 1950.
3. What is the meaning of the underlined word "legacy" in paragraph 3?

A. Someone well known at over the world. B. Someone considered to be an art treasure. C. Something traditional combined with modern dances. D. Something valuable passed down from generation to generation.
4. What is the main idea of the passage?
A. The history of Peking Opera. B. Mei Lanfang’s early stage life. C. The creations of Peking Opera by Mei Lanfang. D. Mei Lanfang’s great contributions to Peking Opera.

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单项选择题

1926年，美国出版两本权威的行政学教科书（），这两本书的出版标志着公共行政学理论体系的形成。

A、怀特的《行政学研究导论》和威洛比的《公共行政学原理》

B、威尔逊的《行政之研究》和威洛比的《公共行政学原理》

C、威尔逊的《行政之研究》和古德诺的《政治与行政》

D、斯坦因的《行政学》和怀特的《行政学研究导论》

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