________ from endless homework on weekends, the students now find their own activities, such

as taking a ride together to watch the sunrise.[      ]

A. Freed      

B. Freeing

C. To free  

D. Having freed

以下措施中有利于限制谐振过电压的为（）。

A、使电网的中性点直接接地运行

B、提高断路器动作的同期性

C、选用灭弧能力强的高压断路器

D、断路器断口加装并联电阻