题目

 Look at the trouble I am in! _______I had followed your advice. A. Only if                     B. If only                   C. Even if            D. So long as 答案：B如图所示电路，R1=7Ω，R2=3Ω，当S闭合时，电压表的示数为21V，当S断开时，电压表的示数为14V．求R3的阻值．