题目

Professional footballers have worryingly poor teeth that could be affecting their performance on the football ground, say dentists. Their study on players at eight clubs in England and Wales, in the British Journal o f Sports Medicine, showed nearly 4 0ut of 10 had cavities （蛀牙) and that athletes often had worse teeth than the general population. Regularly taking sugary foods is one possible explanation. The dentists, from the International Centre for Evidence-Based Oral Health at University College London, examined 187 players' sets of teeth. They found 53% had dental erosion 如图甲所示，理想变压器原、副线圈的匝数比为10：1，R1=20Ω，R2=30Ω，C为电容器．已知通过R1的正弦交流电如图乙所示，则?（　　）A．交流电的频率为0.02 HzB．原线圈输入电压的最大值为200VC．电阻R2的电功率约为6.67 WD．通过R3的电流始终为零