目前各种城域网建设方案的共同点是在结构上采用三层模式，这三层是；核心交换层、业务汇聚层与______。
A. 数据链路层
B. 物理层
C. 接入层
D. 酆网络层
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【B2】
A. in
B. from
C. for
D. by
【9】
A. Cultural Differences
B. Music in the Curriculum
C. Gender Issues
In Smith's view, monopolies
A. will lead the economy to a stop.
B. can hardly realize the checks and balances of competition.
C. may bring about a vicious circle of high production and low demand.
D. will damage the economy completely.
本工程降水井应采用()布置。
A. 单排线状
B. 双排线状
C. 封闭环状
D. 梅花状
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