单选题 To reduce their design complexity, most networks are organized as a stack of (72) , each one built upon the one below it.

【正确答案】 D
【答案解析】[分析] 参考译文：为了减少其设计的复杂性，大多数网络都组织成一个堆叠的层次结构，其中每一层都基于下一层而设计。