关系数据库的关系模型中，采用(21)表达实体集以及实体集之间的联系：关系数据库的3种完整性约束中，(22)规定基本关系的主属性不能取空值。

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When you open a file or run a program in a computer, a window appears on the desktop of your computer. The(67)of the window indicates which program you are currently working in arid its color will become blue if a window is active. When you click on The(68), the window disappears and appears as a button on the taskbar. Clicking on a button in(69)will invoke a specific command.

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A(66)holds enough bits to store a single character while a(67)is a group of bytes.

The HTML is the language used to create(73)for use on the WWW.

Voice-(72)technology converts human speech into a digital code that a computer can under- stand.

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