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【单选题】
在数据库系统中,将满足以下条件的基本层次联系集合称为网状模型:( )。 1允许一个以上的结点无双亲 2仅有一个结点无双亲 3一个结点可以有多于一个双亲 4每个结点仅有一个双亲 5每个结点的双亲是唯一的A.
③和④
B.
②和③
C.
①和④
D.
①和③
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反馈参考答案：
举一反三
【简答题】高效液相色谱定量分析常用 和 。
【单选题】听力原文： All the rescuers found in the mountains of the Sierra de Perija in Venezuela was the scattered wreckage of the plane and the charred corpses of its occupants. There were no survivors. The plane ...
A.
All the passengers were dead.
B.
Both of the plane's engines got lost.
C.
The aircraft exploded and caught fire.
D.
The black box has not been found yet.
【单选题】听力原文： All the rescuers found in the mountains of the Sierra de Perija in Venezuela was the scattered wreckage of the plane and the charred corpses of its occupants. There were no survivors. The plane ...
A.
All the passengers were dead.
B.
Both of the plane's engines' got lost.
C.
The aircraft exploded arid caught fire.
D.
The black box has not been found yet.
【单选题】进行细胞分选时，为了保证分选细胞的生物学活性不受影响，一般要求分选速度至少达到多少 ( )
A.
20000个/秒
B.
10000个/秒
C.
7000个/秒
D.
5000个/秒
E.
3000个/秒
【简答题】33 Chilean miners were trapped nearly a half-mile underground after the mine accident. They had no real hope that they’d ever be found. Luckily, though, the men had Luis Urzua. They were rescued to sa...
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