加工表面的回转轴线与基准面垂直、外形复杂的零件可以装夹在（）上加工。
A.角铁
B.花盘
C.心轴
D.两顶尖
参考答案：B
Slavery was () in Canada in 1833, and Canadian authorities encouraged the slaves, who escaped from America, to settle on its vast virgin land.
A．diluted
B．dissipated
C．abolished
D．resigned
设关系模式R＜U,F＞，其中U{H,I,J,K,L}，若F={H→IJ,J→K,IJK→L,L→H,L→K)，则F的最小函数依赖集Fmin={ (54) }，关系模式R的候选关键字是 (55) 。
（54）处填（）。
A．H→I,H→J,J→K,IJK→L,L→H
B．H→I,H→J,J→K,IJ→L,L→H
C．H→I,H→J,J→K,IJ→L,J→K
D．H→I,J→K,IJ→L,L→H,L→K
