题目

如图所示的电路中，R1、R2均为定值电阻，R3为可变电阻，电源的电动势为E，内阻为r．设理想电流表A的读数为I，理想电压表V的读数为U．当R3滑动触头向图中a端移动时（  ）                                                                                                                          　   A． I变大，U变小                 B． I变大，U变大    C．      I变小，U变大      D． I变小，U变小                                                                                                                                    答案：A

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