加热炉每()应进行一次人工清灰。
A.一个月
B.三个月
C.半年
D.一年
参考答案:C
You can keep the book for another week, but you ______ lend it to others. [ ]
A. needn't
B. mustn't
C. aren't
D. must
拉削的进给运动是由什么实现的?