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 Actuating the CO2 fixed system causes the shutdown of the（）.
单项选择题 A safety outlet is provided on the CO2 discharge piping to prevent（）.
单项选择题 A fire in the galley ALWAYS poses the additional threat of（）.
