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问题 单项选择题

（）有两种形式，一种是机械式的，它是通过钢丝绳及滚轮拉动开关，断开总电源：另一种是与减速、限位开关结构相同的极限开关，动作时，切断上、下接触电源，使电梯停止运行。

A、端站减速安全保护

B、端站限位安全保护

C、端站极限开关保护

D、越程安全保护装置

答案

参考答案：C

填空题

数据流程图描述了数据运动状况，其中的数据及其属性和关系需由 【11】 来定义。
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填空题


The United States counts its population every ten years, and each census reveals that the racial and ethnic mix is changing dramatically, so by the year 2050, the "average" American will not be descended from (1) , but the majority of U. S. residents will trace (2) ancestry to Africa, Asia, the Hispanic world, the Pacific Islands, or the Middle East. Once the United States was a microcosm of European nationalities, (3) the United States is a microcosm of the world. The United States is (4) considered a "melting pot" society by many of its residents. (5) , many people prefer the term "salad bowl." They use this (6) to describe American society. American society will soon be (7) nonwhite. "Melting pot" implies that the different ethnic groups blend together (8) one homogeneous (9) , "salad bowl" (10) that nationalities, like the ingredients in a mixed green salad, retain their cultural (11) . Earlier generations of (12) believed they had to learn English quickly not only to survive but also for (13) .
Now, many immigrant groups do not (14) the same need. Because there are many places in America (15) you can work, shop, get medical care, marry, divorce and die without knowing English. (16) , Chinatown in San Francisco and New York. Also, Los Angeles has many Vietnamese immigrants and immigrants from Mexico. (17) , many immigrant groups want their children to know their (18) culture. Many Hispanics, for instance, want their children to learn (19) English and Spanish and study the Spanish language in school. They are fighting for the (20) to bilingual education in many communities. In many communities they are in the majority.
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