淬硬的零件不能锉削,有型砂、氧化皮的铸件或锻件,需先在砂轮机上打磨,除去型砂及氧化皮后方可锉削。
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冷水机组属螺杆压缩机,压缩机是由一对相互啮合反向旋转的阴、阳转子,在旋转时产生周期性的制冷剂变化来完成()、()、()三个过程。
You need to perform certain data manipulation operations through a view called EMP_DEPT_VU, which you previously created. You want to look at the definition of the view (the SELECT statement on which the view was created.) How do you obtain the definition of the view?()
A. Use the DESCRIBE command on the EMP_DEPT_VU view.
B. Use the DEFINE VIEW command on the EMP_DEPT_VU view.
C. Use the DESCRIBE VIEW command on the EMP_DEPT_VU view.
D. Query the USER_VIEWS data dictionary view to search for the EMP_DEPT_VU view.
E. Query the USER_SOURCE data dictionary view to search for the EMP_DEPT_VU view.
F. Query the USER_OBJECTS data dictionary view to search for the EMP_DEPT_VU view.
重力选煤主要利用物料密度差别进行分选。
A. 正确
B. 错误