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.

（73）处填（）。

A．Conversation

B．Dialog

C．Session

D．Discussion