单项选择题
Which of these is a Layer 2 transport architecture that provides packet-based transmission optimized for data based on a dual （counter-rotating） ring topology？（）
A.DTP B.RPR C.SDH D.CWDM E.DWDM
单项选择题 WhichoftheseLayer2accessdesignsdoesnotsupportVLANextensions？（）
单项选择题 For acceptable voice calls, the packet error rate should be less than（）%
多项选择题 What are three primary activities in the cycle of building an enterprise security strategy?（）
