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问题 单项选择题

下列关于X线管的描述错误的是

A．诊断用X线管的有效焦点均设计为正方形

B．一般X线管常设有两个焦点灯丝

C．更换X线管要用型号相同或具备相同性能的代用管

D．X线管阴极与阳极的距离比整流管两极间距离小

E．管套窗口与管壁窗口应在同一方位上

答案

参考答案：D

解析：X线管阴极与阳极的距离比整流管两极间距离大，有利于电子的加速，提高电子动能，有利X线的产生。

选择题

如图所示的原电池中，下列说法正确的是（　　）

A．正极发生氧化反应

B．该装置能将电能转化为化学能

C．电子由锌片通过导线流向铜片

D．负极反应为 2H++2e-H2↑
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单项选择题

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
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