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6.如果两个实体之间具有m:n联系,则将它们转换为关系模型的结果是___个表。
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7. 在关系数据模型中,把数据看成一个二维表,每一个维表称为一个___
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8.在关系模型中用来唯一标识元组的属性组称为___
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9.在关系数据库中,从关系中找出满足给定条件的元组,该操作可称为___。
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指针式万用表电阻档若无法调至零位,说明(),应更换新电池,否则影响()。
【填空题】
用万用表测量电流时,应让万用表与负载相( ),测量电压时,应让万用表与负载相( )。
【填空题】
测量三相异步电机绕组阻值常使用( ),测量三相异步电机绝缘应使用( )
【填空题】
用数字式万用表量程小于所测参数值时,万用表将显示( )。测量直流电压时,当万用表红表笔与电源负极相接,黑表笔与电源正极相接,万用表显示( )
【填空题】
兆欧表在测量前,指针可以停留在标尺的( )位置,兆欧表在开路时,摇动手柄到额定转速,这时表针应指在( “” ) 位置。
【填空题】
用来测量交流电流的钳形电流表是由()和( )组成的。
【填空题】
直流电的文字符号是( ),图形符号是( )。
【填空题】
交流电的文字符号是( ),图形符号是( )。
【填空题】
中性线()的文字符号是( )、接地的图形符号是( )。
【填空题】
插头插座的图形符号是( 或 ), 原电池或蓄电池的图形符号是( )
【填空题】
电气工程图由( )、( )、 安装接线图 三种图纸构成。
【填空题】
国家标准规定,原理图中各元器件触点,均按( )和 ( )的自然状态下画出。
【填空题】
电气原理图中,组合开关、主令控制器、万能转换开关按手柄处于( )的状态画出, 保护开关处于( )状态画出。
【填空题】
保证电气安全主要有( )措施和( )措施两种。
【填空题】
工厂低压电工在每次操作前都必须用()或()对电线、电器进行验电。
【填空题】
国家标准规定的安全电压等级是6V 、12V、()、()和42V。
【填空题】
机床上的低压照明灯,其电压不应超过( ),在容器内工作时,照明电压应选用( )伏。
【填空题】
电流对人体的伤害形式主要有( )和()
【填空题】
电流对人体的致命电流为()、安全电流为()
【填空题】
人体的触电形式主要有()、()和跨步电压触电。
【填空题】
设备不停电的安全距离,10KV是()米,35KV是()米。
【填空题】
高压设备发生接地故障时,室内不得接近故障点()米以内,室外不得接近故障点()米以内。
【填空题】
凡离地面()以上的地点进行工作时,都视为高空作业,应使用()。
【填空题】
接地种类分为()接地,()接地和重复接地。
【填空题】
重复接地的作用:三相四线系统中,()时,进一步降低发生单相碰壳短路时()。
【填空题】
发现有人触电,应首先(),然后立即()。
【填空题】
发现电器失火紧急事故时,应首先(),然后()。
【填空题】
低压断路器又称自动空气开关,是非常重要的电器,它相当于( )、熔断器 、( )和欠电压继电器的组合。
【填空题】
低压断路器又称自动空气开关,它除具有开关作用外,还具有(、 )、( )、欠压和漏电保护的作用。
【填空题】
组合开关的主要用途是机床电源的( )、用来直接控制小容量电动机的()。
【填空题】
熔断器的主要作用是( ),还可以起到使用电设备与电源()。
【填空题】
熔断器图形符号是( ),文字符号是( )。
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The terms for a Senator and Representative are and years respectively.
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According to the Constitution, a candidate for President must be .
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The first shots of the American dependence were fired in .
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is located on the U. S. –Canadian border between Lake Erie and Lake Ontario.
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The largest river in the U. S. is .
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The U. S. lies in North America, with Canada to the north, Mexico to the south, the Atlantic to its and the Pacific to its .
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The state of is the largest in area of all the states.
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The United States has states on the continent.
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Waiting for Godot is written by .
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Of the following writers, is NOT a Nobel Prize winner.
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Of the following statements, is NOT correct about Virginia Woolf.
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Of the following books, was NOT written by Thomas Hardy.
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is NOT included in the modernist group.
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Of the following statements, is NOT correct in terms of Neo-Romanticism.
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is a representative of English Critical Realism at the turn of the 19th century.
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Of Dickens’ novels, is considered autobiographical.
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is viewed as Romantic poetry’s “Declaration of Independence.”
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