关系运算中花费时间可能最长的运算是______。
A．选择
B．连接
C．并
D．笛卡儿积
参考答案：D
1、It’s___ ___( 星期三 ) today.
2、Do you want to go on a__ ____(旅行 ) to Beijing?
3、There are two __ ____(女人) in the room.
4、The boy always speaks in a l __ ____voice in public.
5、Our teacher are very f__ ___ to us.
接触线补偿下锚的目的何在？
