题目

Every artist knows in his heart that he is saying something to the public. Not only does he want to say it well, but also he wants it to be something which has not been said before. He hopes the public will listen and understand－he wants to teach them, and he wants them to learn from him. What visual artists like painters want to teach is easy to make out but difficult to explain, because painters translate their experiences into shapes and colors, not words. They seem to feel that a certain selection of shapes and colors, out of the countless billions possible, is exceptionally interesting 如图所示，M是小型理想变压器，原、副线圈匝数之比 n1：n2=10：1，接线柱a、b接正弦交变电源，电压u=311sin100πtV．变压器右侧部分为火警系统原理图，其中R2为半导体热敏材料制成的传感器，其电阻随温 度升高而减小；R1为定值电阻．下列说法正确的是（　　）A．电压表V1的示数为31.1VB．变压器副线圈中电流的频率为50HzC．当R2所在处出现火警时，电压表V2的示数变小，电流表A的示数变小D．当R2所在处出现火警时，变压器原线圈的输人功率变大