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【单选题】
以下关于层次数据模型的叙述，错误的是( )。A.
层次数据模型是数据库系统中最早出现的数据模型
B.
它用树形结构表示各类实体以及实体之间的联系
C.
它允许有两个结点无双亲
D.
它的其他结点有且仅一个双亲
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【简答题】Many agers have hobbies. But sometimes these hobbies can get in the way of schoolwork, and parents might worry about their child’s success at school. agers often think they should be allowed t...
【简答题】命名: 图片3.png
【简答题】Sometimes agers ask me ()advice
about what they should do if they want a career like mine one day.A. for B. at C. in D. on
【简答题】一台三相八极感应电动机的电网频率Hz50，空载运行时转速为735转/分，此时转差率为（），转子电势的频率为（）。当转差率为0.04时，转子的转速为（），转子的电势频率为（）。
【简答题】Many agers have hobbies．But sometimes these hobbies can get way of schoolwork. and Parents might worry about their at school．agers often think they should be allowed to practice their hobbies ...
【单选题】According to the passage, we can infer that
A.
teenagers sometimes are promotion targets.
B.
many people don't want to be bothered by ads.
C.
teens can get a free sample by sending a text message.
D.
text message advertisement serves as a lure for teenagers.
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