综合作图--测量电阻：

现有一个电压恒定的电源（电压值未知）、一个电压表（量程大于电源电压值）、一个阻值为R₁的定值电阻、一个单刀双掷开关和若干导线，要利用以上器材测量的待测电阻R₂的阻值．请在方框内设计出实验电路图（电路连接好后实验中不能拆卸）并完成下列步骤．

（1）根据电路图连接实物电路；                               

（2）                                  ；                                   

（3）利用上述测量值算出待测电阻的阻值R₂=     ．

It was in 1949 _____ the People's Republic of China was founded. [ ]

A. when

B. that

C. in which

D. which