短语词组短语coal -mining methods 采煤方法coal surface mining 露天采煤single coal seam mining 单一煤层开采coal resources mining 煤炭资源开采coal pillar mining 带煤柱开采coal mine mining 矿井开采coal caving mining 煤采煤;放顶煤开采;放顶煤opencast coal -mining 露天采煤 返回 coal mining