题目

 如图所示，当交流电源电压恒为220V，频率为50Hz时，三只灯泡A、B、D的亮度相同，若只将交流电的频率改为100Hz，则（    ）A．A灯最暗     B．B灯最暗C．D灯最暗     D．三只灯泡的亮度依然相同  答案： A Mary __________ every day after school, but now she __________ all the time.A. used to play, is used to studyB. used to play, is used to studyingC. is used to play, used to studyD. is used to playing, used to study