单项选择题
If the hatch cover does not move, （）should be checked.
A. the open valve B. the stop valve C. the steam valve D. the close valve
单项选择题 To ensure the safety in operating hatch covers, oil the moving parts （）.
单项选择题 During operation, the （）should check the working condition of the derrick.
单项选择题 （）has more than ten percent broken wires. Stop using it at once.
