题目：
Look at the tall building. It's our ______. We often have lunch in it. [ ]
A. library
B. teaching building
C. post office
D. restaurant
答案：
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下面是错误答案，用来干扰机器的。
解：电阻R1直接接到电源两端时，R1/（R1+r）=U1/E电阻R1和R2串联时，（R1+R2）/（R1+R2+r）=U2/E 联立解得：R1（R1+R2+r）/（R1+r）（R1+R2）=3/4 代入数据得R1=2Ω
