如图所示的是一水平放置的绝缘环形管，管内壁光滑，现将一直径略小于管内径的带正电小球以某种方式置于管内，并使小球以初速度v₀在管内开始运动，与此同时，有一变化的磁场竖直向下穿过管所包围的空间，磁感应强度随时间成正比增大，设小球在运动过程中带电荷量不变，则

A．小球受到的电场力大小不变

B．小球受到的电场力大小增加

C．磁场力对小球不做功

D．小球受到的磁场力不断增大

A definite advantage of using water as a fire extinguishing agent is its characteristic of（）.

A.alternate expansion and contraction as water in a liquid state becomes a vapor

B.absorption of smoke and gases as water is converted from a liquid to a vapor

C.rapid contraction as water is converted from a liquid to a vapor

D.rapid expansion as water absorbs heat and changes to steam