问题 单项选择题

关系模型允许定义3类数据约束,下列不属于数据约束的是()。

A.实体完整性约束

B.参照完整性约束

C.域完整性约束

D.用户自定义的完整性约束

答案

参考答案:C

解析:

关系模型允许定义3类数据约束,它们是实体完整性约束、参照完整性约束及用户自定义的完整性约束。其中前两种完整性约束由关系数据库系统支持,对于用户自定义的完整性约束,则由关系数据库系统提供完整性约束语言,用户利用该语言给出约束条件,运行时由系统自动检查。

阅读理解

The public commonly associates steroid use with big-time athletics. But the drugs may be even more of a menace to teenagers. The synthetic hormones can stunt a young person’s growth by prematurely closing the ends of the long bones in the skeleton. That means a 1.7 metre, 15-year-old high school student who uses steroids “might get bigger but won’t get any taller”, according to a US doctor.

Nearly seven percent of boys in the US try steroids before the end of high school, according to a 1988 study by professors Charles Yesalis and William Buckley, of Penn State University. The estimate comes from a poll of 3,400 seniors in 46 public and private schools across the US.

“Abusers of Steroids did it to improve their appearance and to excel at sports,” Professor Yesalis said. “Parents, teachers and coaches make boys believe that to be an ideal male you need to have these. Then they say: ‘you can’t play games to have fun; you play games to win.’”

“Such attitudes put many high school athletes at high risk of becoming abusers of steroids,” said Mike Gimbel, director of Baltimore County Office of substance Abuse. “For these athletes, the pressure to perform is incredible,” he said. “It was inevitable that it would seep down to high school level.”

小题1:The word “big-time” in Paragraph 1 most nearly means “________”.

A.long time

B.good time

C.top rank

D.modern小题2:Why are steroids even more of a threat to teenagers?

A.Because they make teenagers grow too tall.

B.Because they make teenagers get too big.

C.Because they prevent teenagers from getting taller and taller.

D.Because they give teenagers too much pressure.小题3:Which of the following statements is implied but not mentioned?

A.School athletes have too much pressure.

B.Steroids can help people have lots of muscles.

C.The pressure chiefly comes from parents, teachers and coaches.

D.Girls are not interested in steroids.小题4:Which of the following is a good title for this passage?

A.Drugs and Sports

B.The Pressure to School Boys are Too Much

C.School Boys are Interested in Drugs

D.School Athletes Risk Damage from Drugs

多项选择题