单选

在OSI参考模型中，物理层的功能是(25)等。实体在一次交互作用中传送的信息单位称为(26)，它包括(27)两部分。上下邻层实体之间的接口称为服务访问点(SAP)，网络层的服务访问点也称为(28)，通常分为(29)两部分。

A．建立和释放连接
B．透明地传输比特流
C．在物理实体间传送数据帧
D．发送和接收用户数据

试题出自试卷《软件水平考试（中级）网络工程师上午（基础知识）试题模拟试卷24》

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