principal mining level

例句：

1. 1. The company has decided to increase its investment in the principal mining level.
   该公司决定增加其在主要开采阶段的投资。
2. 2.The discovery of a new mineral deposit is a fundamental step towards sustainable development.
   新矿藏的发现是实现可持续发展的基本步骤。
3. 3. The decision to prioritize the primary mining level was made based on market analysis.
   根据市场分析做出优先开采主要开采级别的决定。