在概念模型中，惟一标识实体的属性集称为( )。
A．元组
B．关系
C．主码
D．主属性
参考答案：C
解析：[分析] 如果在一个关系中，存在多个属性(或属性组合)都能用来惟一标识该关系的元组，这些属性(或属性组合)都称为该关系的候选码或候选键。在一个关系的若干个候选码中指定一个用来惟一标识该关系的元组，这个被指定的候选码称为该关系的主码或主键。
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| Hello, everyone ! My name is Li Qiang. I'm from Yancheng, Jiangsu, China. Now I live in America. I live here because my __1__ work here. My father __2__ Chinese and my mother is an office worker. I am in Grade 7 this year. The schools here are __3__ from the schools in Yancheng. Our grade has 10 __4__ . Each of the classes has about 15 students. The school day starts __5__ here. We have our first lesson at 9:00 a.m. I have lunch at school with my friends. We can go __6__ at 3:30 in the afternoon. After school, I __7__ lots of time playing basketball. I __8__ do many things that I would like to do. We don't have much __9__ to do, and my homework is usually interesting. We don't do many exercises about the lessons. Sometimes I don't think it's a good thing, because many of my classmates don't do well in __10__ lessons. | |||
| ( ) 1. A. friends ( ) 2. A. teach ( ) 3. A. different ( ) 4. A. classes ( ) 5. A. at ( ) 6. A. to sleep ( ) 7. A. do ( ) 8. A. must ( ) 9. A. work ( ) 10.A. they | B. parents B. teaches B. far B. teachers B. late B. to school B. need B. can B. things B. them | C. teachers C. teaching C. hard C. students C. early C. to work C. want C. can't C. homework C. their | D. sisters D. to teach D. interesting D. schools D. in D. home D. spend D. should D. exercise D. theirs |