关系的基数是衡量关系复杂性的指标之一，又被称为关系的约束。一个实体在关系中的基数定义了在关系中其他实体实例确定的情况下，该实体实例可能参与关系的数量
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Understanding a conversationDirections: Listen to a conversation and choose the best answer from the four choices provided for each question.[音频]
A. To work as club president.
B. To lend some money to him.
C. To let him copy her homework.
D. To complete assignments for him.
Why did the woman refuse to help the man?
A. She was arrogant
B. She was afraid of him
C. She thought there might be a trouble
D. She disliked him
2
A. Because she doesn’t have time.
Because she is afraid to get in trouble.
C. Because she would be late with her own homework.
D. Because she doesn’t understand the learning material.
3
A. Learning.
Being honest.
C. Passing exams.
Doing homework.
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