单项选择题 When water is used to fight a fire on board a ship，the effect of the weight of the water must be taken into account. How much sea water will increase the weight displacement by one ton?（）

单项选择题 When using carbon dioxide to fight a fire on a bulkhead，the CO2 should be applied（）.

单项选择题 When using a high-velocity fog stream in a passageway，the possibility of a blow back must be guarded against. Blow back is most likely to occur when（）.