龙门架的卷扬机安装后，二者之间的距离应不小于卷扬机滚筒宽度的（）倍。
A.10
B.15
C.20
D.25
参考答案：C
China's icebreaker Xuelong ______ by heavy floes since Friday after rescuing passengers on a Russian vessel stranded in Antarctica.
A．has been trapped
B．had been trapped
C．has trapped
D．was trapped
机组磁极对数为12对，问频率50Hz时该机组转速为多少？
