采掘工作面的进风流中,按体积计算,氧气浓度不得低于()。
A.16%
B.18%
C.20%
参考答案:C
肾经的井穴是()。太溪位于()处。
One of the limitations of foam as an extinguishing agent is that foam().
A.cannot be made with salt water
B.is heavier than oil and sinks below its surface
C.is corrosive and a hazard to fire fighters
D.conducts electricity