题目

Some years ago on a hot summer day in south Florida a little boy decided to go for a swim in the old swimming hole behind his house. In a hurry to dive into the cool water, he ran out of the back door, leaving behind shoes, socks, and shirt as he went.He flew into the water, not realizing that as he swam toward the middle of the lake, a crocodile was swimming toward him. His mother in the house was looking out of the window and saw the two. In great fear, she ran toward the water, shouting to her son as loudly as she could. Hearing her voice, the little boy became alarmed and made a U-turn to 如图所示，质量为1 kg的A物体重叠在质量为2 kg的B物体上， A、B之间的动摩擦因数为0.4，B和地面间的动摩擦因数是0.1．A的带电量为，B不带电．A、B是绝缘的．整个装置处于水平向右的匀强电场中，电场度从0开始逐渐增大，开始时A、B两物体均处于静止状态．取，最大静摩擦力可视为与滑动摩擦力相等．则下列判断正确的是（   ）A．A先相对B动起来，B后相对于地动起来B．当电场强度增大到后，A、B一定发生相对滑动C．A、B运动的共同加速度可以达到D．A、B运动的共同加速度可以达到