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问题 不定项选择

关于摩擦起电和感应起电的实质，下列说法正确的是[ ]

A．摩擦起电现象说明了机械能可以转化为电能，也说明通过做功可以创造电荷

B．摩擦起电说明电荷可以从一个物体转移到另一个物体

C．感应起电说明电荷可以从物体的一个部分转移到另一个部分

D．感应起电说明电荷从带电的物体转移到原来不带电的物体上去了

答案

BC

选择题

—How much does it ________ you to buy the bicycle ?

—About three yuang.

A．pay

B．spend

C．cost

D．take
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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.

13（）

A.special

B.dependent

C.similar

D.dissimilar
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