In the open systems interconnection (OSI) reference model, "layer" means one of seven conceptually complete, (71) arranged groups of services, functions, and protocols, that extend across all open systems.
Application layer provides means for the application (72) to access the OSI environment. Presentation layer provides for the selection of a common syntax for representing data. (73) layer provides the means necessary for cooperating presentation entities to organize and synchronize their dialog and to manage their data exchange. Transport layer provides a reliable end-to-end data transfer service. Network layer provides for the entities in the transport layer the means for transferring blocks of data, by (74) and switching through the network between the open systems in which those entities reside. Data link layer provides services to transfer data between network layer entities. Physical layer provides the mechanical, electrical, functional, and procedural means to establish, maintain and (75) physical connections for transfer of bits over a transmission medium.
（75）处填（）。
A．release
B．disconnect
C．free
D．terminate
参考答案：A
解析：
参考译文
在开放系统互连(OSI)参考模型中，所谓“一层”是指：对跨所有开放系统的服务、功能和协议，按层次方式安排在概念上完备的7个群组之一。
应用层为应用过程提供接入开放系统互连环境的手段。表示层为表示数据的公共语法提供选择。会话层为协调各显示实体提供必需的手段，用于组织和同步这些实体的对话，并管理其间的数据交换。传输层提供一种可靠的端对端数据传送服务。网络层通过各实体所驻留的各开放系统之间的网络上的路由选择与交换，来为传输层中的这些实体提供传送数据块的手段。数据链路层为在网络层实体之间的数据传送提供服务。物理层提供机械的、电气的、功能的和过程的手段，为在某一传输媒体上传送比特流，建立、维护与释放物理连接。