在图（a）所示的电路中，电源电压不变，电阻R₁的阻值为20欧．闭合电键S，电流表A₁和A₂的示数分别为0.3安和0.9安．求：

（1）电源电压U．

（2）此时电阻R₁消耗的电功率P₁．

（3）现用R₃去替换R₁，闭合电键S，当滑动变阻器的滑片移到某个位置时，电流表A₁和A₂的指针都指在如图（b）所在的位置，求此时滑动变阻器R₂连入电路的阻值．

For many years, people () electric cars. However, making them has been more difficult that predicted.

A. had dreamed of

B. have dreamed of

C. dreamed of

D. dream of