Within the seven coal forming periods restricted to from Late Carboniferous to early Late Permian,there are seven coal groups and more than 20 seams defined in the whole area,of which seams B 3,D 2and G 7 1 are economically important.是什么意思_Within the seven coal forming periods restricted to from Late Carboniferous to early Late Permian,there are seven coal groups and more than 20 seams defined in the whole area,of which seams B 3,D 2and G 7 1 are economically important.在線(xiàn)翻譯_英語(yǔ)_讀音_用法_例句