服役期未满，因下列哪些原因可提前退出现役？（）

A.因战、因公负伤致残

B.部队编制员额缩减

C.根据自己意愿想退出现役的

D.国家建设需调出部队

E.患精神病未治愈的

Networks can be interconnected by different devices. In the physical layer, networks can be connected by （） 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 （） , 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 （） network layer, the router may be able to translate between the packet 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 （） . As an example, gateways between Internet e-mail and X.400 e-mail must （） the e-mail message and change various header fields.

They can accept（）, examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process.

A．frames

B．packets

C．packages

D．cells