在研磨过程中的切削作用、研磨效率、研磨精度和表面粗糙度，都与（）有密切关系。
A.磨料
B.研磨液
C.研具
D.工作
参考答案：A
The meeting _______ by the time I got there yesterday. [ ]
A. was on
B. has been on
C. had begun
D. has begun
在初步确定年调节和多年调节水电站装机规模时，其经济利用小时数常取（）h。
A.1500～2500
B.2500～3500
C.3500～4500
D.4500～5500
