Which categories are the top-level categories, and which are sub-categories?
哪些类别是顶级类别，哪些是子类别？
Sub-level caving is an efficient and safe mining method.
无底柱分段崩落采矿法是一种高效安全的采矿法。
The control of water level and combustion sub-system is realized.
实现对水位和燃烧子系统的控制。
The measure sub-element , the phase-comparator , and the power level are introduced.
对检测环节、鉴相器、功率级做了介绍；
The measure sub-element , the phase-comparator , and the power level are introduced.
对检测环节、鉴相器、功率级做了介绍；
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