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.

（12）是（）

A.frames

B.packets

C.packages

D.cells

下列关于计算机的叙述中，不正确的一条是( )。

A．运算器主要由一个加法器、一个寄存器和控制线路组成

B．一个字节等于8个二进制位

C．CPU是计算机的核心部件

D．磁盘存储器是一种输出设备