单项选择题
The guard, fencing or shielding for moving parts or machinery can be removed for repair only when the machinery it belongs to（）
A.stays at rest B.is working stably C.is worn out D.is in good operation
单项选择题 Before starting an engine by air, the engine must（）
单项选择题 The oil pressure and flow from each bearings can be checked when（）
单项选择题 When preparing an engine for water temperature（）
