在工件上既有平面又有孔需要加工时,可采用()。
A.粗铣平面→钻孔→精铣平面
B.先加工平面,后加工孔
C.先加工孔,后加工平面
D.任一种形式
参考答案:B
They have been skating ______. [ ]
A. for three hours ago
B. since three hours ago
C. three hours go
D. since three hours
怎样抑制光污染?