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.
15（）
A.analyze
B.parse
C.delete
D.create
参考答案：B
解析：
涉及是网络的相关知识，参考译文如下。
网络可以用不同的设备互联，在物理层用中继器或集线器互联，这些设备只是在相同的网络之间传送比特串；在上面的数据链路层可以使用网桥或交换机，这些设备接收数据帧，检查MAC地址，并可以实现少量的协议转换，把数据帧转发到不同的网络中；在网络层我们使用路由器连接两个网络，如果两个网络的网络层不同，路由器能够转换分组格式；在传输层我们使用传输网关，它可以在两个传输连接之间建立接口；最后在应用层应用网关实现消息语法之间的翻译。例如，在Internet邮件和X.400邮件之间的网关可以对邮件报文进行语义分析，并修改报文的各个报头字段。