单项选择题
A.3m B.5m C.10m D.20m
A.50~100mm B.100~150mm C.150~250mm D.200~300mm
A.煤底实茬 B.实底岩石 C.结实木垫 D.金属垫脚
A.2m B.3m C.4mm D.5m
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