Astute service providers realize that the continued support of legacy X.25 and asynchronous network element using separate operations network is cost(16).For example,the maintenance of multiple networks can require additional staff. Often, this staff must be trained on multiple vendor technologies, sometimes requiring parallel groups specializing in each (17) .Hence, additional maintenance procedures must be maintained and administrative records are (18) .The duplication of transport facilities to carry (19)network traffic is an inefficient use of resources. And not surprisingly, more technologies installed in a central office means the necessity for more physical space, and an increase in power connections and power consumption migration of these (20) network elements to IP-based DCns is alogical strategy.空白处（20）应选择（）
A、raditional
B、dominancy
C、redundancy
D、legacy