My Deratization Exemption Certificate will become ()on the 16th of September.
A. due
B. expire
C. lawful
D. non-effective
During (), the starting air valves should be closed to prevent the engine from turning.
A. overhaul
B. running
C. blowing down
D. All of the above are correct
信息系统的生命周期可以分成()。
A. 系统规划
B. 系统开发
C. 系统运行与维护
D. 系统更新
不同的导轨的当量滑动摩擦因数是不同的,其中fv值为f的是()导轨。
A. 矩形导轨
B. 燕尾形导轨
C. 三角形导轨
D. 圆柱面导轨