在关系数据库中，实现主键值惟一标识元组的作用是通过( )。
A．实体完整性规则
B．参照完整性规则
C．用户定义的完整性规则
D．强制不能为空值
参考答案：A
_____ the morning of December 1st, 2006, the 15th Asia Games _____ in Doha (多哈). [ ]
A. In; held
B. On; was held
C. At; held
D. For; were held
夹板有几种形式？
