题目

--Is my answer to the math problem fight? ---Let me consider it carefully            I give you an answer.A．beforeB．afterC．sinceD．as soon as 答案：A解析:略如图所示，电源电压不变，R1=3Ω，R2=6Ω．当S1，S2都断开时，电流表的读数为0.3A．当S1，S2都闭合时，电流表的读数为1.5A，求电源电压和R3的阻值．