应用最为广泛的镗床专用夹具是()。
A.样板
B.镗套
C.镗模
D.镗杆
参考答案:C
You are requested at an English evening to be held at the school auditorium on Friday, at 7.00 pm.
A.be present
B.to present
C.presenting
D.presented
根据某些条件对一个关系做水平分解,选择符合条件的元组组成一个新的关系,这样的操作称为
A.投影
B.选择
C.连接
D.自然连接