一个关系中属性个数为1时，称此关系为
A．对应关系
B．单一关系
C．一元关系
D．二元关系
参考答案：C
解析：【命题目的】使学生加深对关系模型数据库基础知识的记忆。【解题要点】在关系模型数据库中，基本结构是二维表，这种二维表称为关系。关系的列称为属性，一个具有N个属性的关系称为N元关系。【考点链接】各种关系查询语言与过程性语言和非过程性语言的关系，过程性语言的特点。
| The "Karat" marking on jewelry tells you what proportion of gold is mixed with other metals, ff 14 parts of gold are mixed with 10 parts of base metal, the combination is called 14-Karat (14K) gold. The higher the Karat rating, the higher the proportion of gold in the object. The lowest Karat gold that can be marketed in the United States is 10-Karat gold. Jewelry does not have to be marked with its Karat quality, but most of it is. ff there is a Karat quality mark, next to it must be the U.S. registered trademark of the person or company that will stand behind the mark, as required by the National Gold and Silver Stamping Act. |
Gold which is 10 Karats m proportion
A．represents the highest grade of gold in the U.S.
B．cannot be sold in the U.S.
C．never carries a Karat quality mark.
D．represents the lowest-grade gold marketable in the U.S.