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设A是n阶实对称矩阵，如果A2=O，证明A=O．并举例说明，如果A不是实对称矩阵，上述命题不正确．

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Part A You will hear 10 short dialogues, For each dialogue, there is one question and four possible answers. Choose the correct answer—A, B, C or D, and mark it in your test booklet. You will have 15 seconds to answer the question and you will hear each dialogue ONLY ONCE. Now look at Question 1. What does the man mean

A. The park is nearby.
B. He doesn’t know about the park.
C. There is no parking here.
D. He doesn’t understand the question.

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下列要素中，不属于DFD的是 (17) 。当使用DFD对一个工资系统进行建模时， (18) 可以被认定为外部实体。

A. 加工
B. 数据流
C. 数据存储
D. 联系

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Why did the lower-class white people in the United States treat Chinese immigrants with hostility

A. Because they didn’t understand their language.
Because they saw them as a source of threat to their jobs.
C. Because their culture was very different from theirs.
D. Because they had very different appearances.

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逆向工程过程的抽象层次是指可从源代码中抽取出来的设计信息的精密程度。抽象层次分为四层，其中“最低层”抽象能够导出过程的设计表示文档，“低层”抽象能够导出程序和数据结构信息，“中层”抽象能够导出 (1) ，“高层”抽象能够导出 (2) 。

A. 实体关系模型
B. 模块结构图
C. 完全的数据流图
D. 全部文档信息

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