题目

如图所示，在水平面上的箱子内，带异种电荷的小球a、b用绝缘细线分别系于上、下两边，处于静止状态．地面受到的压力为N，球b所受细线的拉力为F。剪断连接球b的细线后，在球b上升过程中地面受到的压力 ( ) A . 大于N+F B . 等于N C . 等于N＋F D . 小于N 答案：ALi Ming told me once he had his arm broken while playing basketball. ________ was that? [　　] A．How long agoB．Since when C．For how longD．How soon