题目

 Do remember to wash the vegetables ___________. A. as clean as possibly           B. as cleanly as possible C. as clean as possible           D. as clean as you do 答案：C4．如图所示的电路中采用新的干电池作为电源，电阻R1的阻值为10欧．开关S闭合时，电流表A的示数如图中的甲所示．开关S断开时，电流表A的示数如图中的乙所示．求：（1）电源电压U；（2）电阻R2的阻值．