第19题: [单项选择]中频电疗法的频率范围为() A. 0~1kHz B. 1~100kHz C. 100~200kHz D. 500~1000kHz E. 200kHz以上 参考答案:B
第20题: [单项选择]All the lights ______ before we left the office. A. has been turned off B. have turned C. had been turned off D. had turned off 参考答案:C 答案解析:[解析] 主句的动作发生在从句的动作之前,表示过去某时刻前发生的动作,即“过去的过去”,所以要用过去完成时;主谓之间是被动关系,所以应用被动语态。