许多系统是在三个主要层次上设计的：系统、子系统和组件，如 图 1 所示。
Many systems are designed at three primary levels: system, subsystem, and component, as shown in Figure 1.
教学设计对象包括教学活动、教学环境、知识传递，其中教学活动处于三个层次的核心地位。
The objects of instructional design consist of teaching activity, teaching environment and knowledge transmission of which teaching activity is central.
接着详细介绍了产品设计中的三个层次和产品设计企划的基本要点。
Thirdly, knowing the three levels of product design help us analyzing the basic point of product design planning.
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