HXD3型电力机车六组（）电路是相对独立，分别为六个牵引电动机提供交流变频电源。
参考答案：牵引变流器
_____ 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
普通粘土砖的强度等级是怎样划分的？
