现阶段公认的板块运动的动力是（）。
A、海洋洋流
B、地幔对流
C、火山运动
D、能量释放
参考答案：B
The workers were made ______ more than 12 hours a day.[ ]
A．work
B．working
C．to work
D．works
钻削速度就是指钻头每分钟的转数。
