题目

Experts believe people can waste less food by shopping only when it is necessary. A . why B . where C . that D . what 答案：C如图所示的电路中，R1=20Ω，电路总电阻为12Ω，电流表示数为0.3A，请计算：（1）电阻R2的阻值；（2）电源电压；（3）通过R2的电流．