单项选择题
A.frequent opening of hatches to permit entry of fresh air
B.regular checks of bilge levels
C.use of ear plugs to dampen engine noise
D.frequent flushing of the water spray system with fresh water
单项选择题 After each reading of an oxygen indicator, the instrument should be purged with（）.
单项选择题 After extinguishing a fire with C02,it is availabe to（）.
单项选择题 After putting on a self-contained breathing apparatus, you open the air supply and hear a continuous ringing of a bell.What does this mean?（）.