单项选择题
After each reading of an oxygen indicator, the instrument should be purged with（）.
A.CO2 B.fresh air C.the tested compartment's air D.water
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?（）.
单项选择题 After using a C02 portable extinguisher,it should be（）.
