单项选择题
A.18900mmB.19900mmC.20900mmD.21900mm
A.定滑轮组B.平衡滑轮组C.动滑轮组D.定滑轮组上钢丝绳
A.1200mmB.1300mmC.1400mmD.1500mm
A.160tB.170tC.180tD.190t
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