单项选择题
To ensure the safety in operating hatch covers, oil the moving parts （）.
A. monthly B. every three months C. once monthly D. every two months
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.
单项选择题 It is flammable cargo. Keep it away from the （）.
