... information is uncertain.B7#,B8# ——applicable to stable,vitreous,low abrasive,hard and slippery stratum.B9#,B10# ——applicable to ... information is uncertain.C7#,C8# ——applicable to stable,vitreous,low abrasive,hard and slippery stratum.C9#,C10# ——applicable to ... special requirements.Recommendation:When hardness of stratum is high, low grade drilling bits are recommended to ensure matching of stratum.( ...