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?（）.
A.The unit is working properly B.The face mask is not sealed properly C.The air bottle needs to be refilled D.The air supply hose has a leak
单项选择题 After using a C02 portable extinguisher,it should be（）.
单项选择题 All dunnage planks and mats must be sent to board（）commencement of loading.
单项选择题 All echo-sounders can measure the（）.
