《语音篇》为基础阶段辅助教材，其余三篇主干教材依次按基础、初级、中级三个等级设置。

The Phonetics book is a supplementary one at the starter's level, while the other three are designed for learners at starter's, elementary and intermediate levels respectively.

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进而按照通达程度将公路网划分为四个等级：主干线、次主干线、支线和连接线，并分别对四个层级进行了功能定位；

Then divides the highway network into four levels: main trunk line, sub-trunk, feeder and connecting line, and also positions functions of the four levels.

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