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.

A．repeaters

B．relays

C．packages

D．modems

物质的性质决定了物质的用途．以下做法是利用物质的物理性质的是（　　）

A．在食品包装袋中填充氮气以防食品腐败

B．氢气常用作清洁能源

C．医院常在蓝色钢瓶中备有液氧供医疗急救

D．用活性炭去除水中的颜色和异味