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
正确答案：C
在穿上自给式呼吸设备后你打开空气供应开关并听到连续响起的笛声这意味着什么？空气瓶需要被重新充装空气
