题目内容

在数据库系统中，我们把满足以下两个条件的基本层次联系的集合称为层次模型 (1) 在数据库系统中，将满足以下两个条件的基本层次联系集合称为网状模型 (2) 。Ⅰ. 允许一个以上的结点无双亲 Ⅱ. 有且仅有一个结点无双亲Ⅲ．其它结点有且仅有一个双亲 Ⅵ．有且仅有一个结点有多个双亲Ⅴ. 有一个结点无双亲 Ⅳ．一个结点可以有多于一个双亲 1（）

A. Ⅰ和Ⅴ
B. Ⅱ和Ⅲ
C. Ⅲ和Ⅳ
D. Ⅰ和Ⅱ

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