如图A、B是两个横截面积不同的竖直圆筒，其中各有一个活塞，它们距离筒底的高度均为h₀=0.2m，质量分别为m₁=1kg，m₂=2kg，活塞下面盛有温度相同且不变的同种气体，活塞上面为真空，两个圆筒的底部是由细管相连通的．如果在A活塞上面再加放一个质量为m=0.5kg的物体，稳定后两活塞的高度差为______m．

A “fifteen-pound” CO₂ extinguisher is so called because（）.

A.there are fifteen pounds of CO₂ in the container

B.the container，when full，weighs fifteen pounds

C.the pressure at the discharge nozzle is l5 psi

D.the empty container weighs fifteen pounds