题目

Theteacher,with6 girls and 8 boys of her class, __________visiting a museum when the earthquake struck.    A. was           B. were               C. had been          D. would be 答案：A若把两定值电阻R1,R2以某种形式连接起来与电源接通,R1消耗的电功率是9W;若把此二电阻换成另一种连接后,仍与该电源接通,则R1消耗的电功率为16W且通过R2的电流为4A.那么电源电压和R1,R2的阻值各是多大?(电源电压保持不变)