题目

 We __________ in the same school since three years ago.    A. study          B. were studying          C. will study                 D. have studied 答案：D实验桌上有如下器材：电源一台；调好的电流表一块、电压表一块；开关一个、滑动变阻器一个；阻值已知但大小不同的定值电阻六个和导线若干．请你从中选择器材，设计一个实验证明：“当电阻两端的电压保持不变时，通过电阻的电流跟电阻成反比”．（1）画出实验电路图；（2）写出主要实验操作步骤．