Networks can be interconnected by different devices. In the physical layer, networks can be connected by (11) or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept (12) ,examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have (13) network layer, the router may be able to translate between the packer formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message (14) . As an example, gateways between Internet e-mail and X.400 e-mail must (15) the e-mail message and change various header fields.

13（）

A.special

B.dependent

C.similar

D.dissimilar

全科医疗作为以门诊为主体的基层医疗保健服务为（）

A.以控制人群卫生需求为主要目的

B.为患者提供以诊断治疗为主的服务

C.居民就医最先接触的专科服务（首诊服务）

D.社区居民健康促进性的社区卫生服务

E.群防群治性服务