题目

如图所示为电热毯的电路图，电热丝接在u=311sin100πt V的电源上．电热毯被加热到一定温度后，通过装置P使输入电压变为图（b）所示波形，从而进入保温状态．若电热丝电阻保持不变，此时交流电压表的读数是（     ）                                                                                                                                                                                                 　   A． 110 V        B．156 V               C． 220 V       D． 311 V                                                                                                                                        答案：B

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