粗铣时，限制进给量提高的主要因素是（）。
A、切削力
B、表面粗糙度
C、加工余量
D、加工刚度
参考答案：A
The little kid ________ to bed twenty minutes ago, but now he's running around the kitchen.[ ]
A. put
B. has put
C. was put
D. was putting
进入灾区前必须检查氧气呼吸器，氧气压力不得低于多少？
