题目

两个半径为R的带电球所带电量分别为+Q1和―Q2，当两球心相距3R时，相互作用的静电力大小为： A．      B．C．       D．无法确定高 答案：B 选词填空。 go off,   run off,   by the time,   on time 1. _____ I got back to school, the bell had already rung. 2. My alarm clock didn't _____ this morning. 3. I got up late and I had to _____ to the bus stop. 4. She couldn't get to school _____ because she missed the early bus.