直流电机换向极绕组应与电枢绕组（），且绕组应装在换相极铁心上。
A、串联
B、并联
C、混联
D、串并联均可
参考答案：A
It is grateful for the girl ____ with her heavy luggage. [ ]
A. to help
B. to be helping
C. to be helped
D. be helping
