E-R模型用于数据库设计的哪一个阶段
A．需求分析
B．概念结构设计
C．逻辑结构设计
D．物理结构设计
参考答案：B
解析： 数据库的设计可以分为6个阶段：需求分析、概念结构设计、逻辑结构设计、物理结构设计、数据库实施和数据库的运行和维护。其中数据库的概念设计的任务是产生反映企业信息需求的数据库概念，即概念模型。设计数据库概念模型最著名、最常用的方法是“实体-联系方法”，即E-R模型。因此E-R模型用于数据库设计的概念结构设计阶段，正确答案为选项B。
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| There are four seasons in a year. They are spring, summer, fall and winter. 1________ is the first season of the year. It is getting 2________ and flowers begin to open. Trees and grass begin to turn 3________. It's usually rainy, people like to fly kites. Then it is summer and it is the 4________ season of the year. The days get 5________, so we can play in the afternoon. 6________ is the third season. It's very beautiful. It's not too hot and not too cold. There is usually 7________ in fall and leaves begin to turn yellow and come down. Many people like to have a picnic in the park. The last season of the year is winter. People usually don't like to go 8________ because it is cold. And there is sometimes 9________ in winter. Then we play with it. But it is the 10________ of the year. So we have to wear heavy clothes such as winter jacket in winter.
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